Automatic map compilation system

ABSTRACT

An automatic map complication system, for deriving information from a pair of stereoscopic photographs by scanning portions thereof which represent homologous imagery comprising: 
     first means for scanning at least portions of a pair of stereoscopic photographs and developing video signals in accordance therewith; 
     second means, including computing means, operable in a sequence of cycles of operation, for estimating for successive cycles of operation, on the basis of at least photogrammetric data related to said pair of photographs, the portions of said photographs which represent homologous imagery and controlling said first means to synchronously scan said portions; and 
     third means responsive to said video signals for comparing said video signals therein and producing error signals which represent the degree of error in said second means in estimating said portions of said photographs as representing homologous imagery, said second means including means responsive to said error signals for updating the estimate of the locations of portions of said photographs, which represent homologous imagery, adjacent the scanned portions.

The present application is a continuation in-part of copendingapplication Ser. No. 240,635, filed Nov. 28, 1962, and assigned to thesame assignee as the present invention.

This invention relates to automatic compilation systems and moreparticularly to a system for automatically compiling maps byautomatically deriving topographic information from a pair ofstereoscopic photographs of actual terrain, and producing anortho-graphic projection photo and an elevation or altitude charttherefrom.

There are a number of seemingly unrelated fields which share a commonproblem, that of deriving or transferring information relating to threedimensional aspects of particular surfaces from two dimensional aspectsof the same surfaces. Some obvious examples may be found in the fieldsof map making, surveying, machine tool control, pattern and die making,and highway construction, to name but a few. In some of these cases,automatic reproduction techniques have been employed which control acopying and recording mechanism from the contour of an actual model.This, of course, is not feasible in some fields, such as topographic mapmaking, in which the requisite information cannot be derivedautomatically from the actual terrain with any degree of practicability.Even in those cases where previously known automatic reproductiontechniques are feasible:, they are generally limited by some kind ofdirect coupling between the contour model and the copying or recordingmechanism. In any case, the known reproduction techniques may beconsiderably improved with resulting substantial savings in time,effort, money and equipment required, through resort to photographicreproduction of the particular surface with respect to which informationis desired, provided there is some system for simply, precisely, andautomatically deriving the desired information from photographs of theoriginal surface.

The need for such a system has been particularly acute in land surveyingoperations, which are laborious, costly and time consuming when used forobtaining terrain information for the production of maps. In recenttimes, aerial photographs have been widely used for obtaining thedesired terrain information, thereby greatly simplifying the fieldoperations. However, the process of extracting the desired data from thephotographs has also been a tedious and time consuming task.Conventionally, aerial photographs taken in an airplane flying over aparticular region in a predetermined pattern have been stereoscopicallyviewed in order to derive actual terrain information from the data inthe photographs. The Kelsh plotter for example, is an instrument whereina virtual image of the overlapping area of the two photographs shows theterrain in true relief. Measurements made by skilled personnel on thisimage yield the positions of the various elevation contours. Anautomatic Stereomapping system, which is the subject matter of copendingapplication Ser. No. 199,797, filed Jun. 4, 1962, represents onesolution to the automation of this task. The system disclosed thereinrepresents a considerable improvement over previously known processes byautomating conventional photogrammetric techniques. However, the systemutilizes a modified Kelsh plotter, thus necessarily involving equipmentwhich is limited in speed and versatility by virtue of its mechanicalprojection system and related apparatus. Furthermore, although thedegree of dependence upon skilled personnel in the production of thedesired mapping and contour information is materially reduced in theautomatic stereomapping system from that which had previously beenrequired, the operation of the system still requires considerableoperator attention, particularly in making the initial adjustments priorto beginning the automatic processing of the information provided in thestereoscopic photographs.

The system of the present invention departs from previously knownsystems and techniques known in the art of photogrammetry and representsa new and novel solution to the automatic extraction of terraininformation from stereoscopic photographs; it operates with higheraccuracy and greater speed than any systems hitherto used. Further, thenovel system of the present invention extracts altitude data of theterrain in the photographs in digital form which may be stored and usedin applications other than map making. Although the present inventionwill be described in the context of map making and more specifically inproducing orthographic projections of the terrain in the stereoscopicphotographs and altitude charts thereof, it should be borne in mind thatthe principles of the present invention are not limited thereto; thespecific examples used hereafter are presented four explanatory purposesonly.

It is therefore a general object of this invention to provide animproved system for automatically deriving suitable contour informationfrom stereoscopic photographs.

It is a further object of this invention to provide an automaticprofiling system operating from photographs used to provide inputinformation, which system is faster and more versatile than previouslyknown arrangements.

It is a specific object of this invention to provide an automatic: mapcompilation system which is easier to use and whose operation demandsless attention from skilled operators than previously known compilationsystems.

It is also a specific object of this invention to provide an automaticmap compilation system which derives terrain information of highaccuracy.

A further object of this invention is to provide an automatic mapcompilation system which is so arranged as to be adaptable to anticipatefuture requirements and further developments.

In brief, the system of the present invention utilizes a digitalcomputer to control an analog system wherein a pair of stereoscopicphotographs are scanned, and signals modulated by the terrain imagerydetails in the photographs are processed to determine the altitude ofthe area scanned. The system may also provide specific outputinformation in the form of an altitude chart showing contour intervalsof the terrain area in the photographs. The system may further produce anew photograph or orthophoto wherein the imagery of the terrain appearsin true orthographic projection. The processing of the photographs isaccomplished by using analog techniques in combination with a smalldigital computer, which is used to perform the calculations involved inrelating the terrain three dimensional coordinates with the twodimensional coordinates of the photographs and to provide all necessarylinks between the analog and the digital subsystems.

In one particular arrangement in accordance with the invention, a pairof stereoscopic photographs or positive transparencies, known asdiapositives, made from original film negatives, which for convenienceare assumed to be included in the term "photographs" used hereinafter,are positioned on a table along with a pair of photosensitive filmsheets on which the orthophoto and altitude chart are to be printed. Thetable is movable along two coordinates so as to permit analysis of thephotographs in a predetermined pattern. Profiling is achieved by movingthe photographs under a pair of flying spot scanners, which, throughassociated lens systems, project individual light beams to selectedincremental portions of the respective photographs. The orthophoto andthe altitude chart film sheets, which are moved with the photographs,are in proximity to two cathode ray tubes which are controlled to printout the desired information. By having the photographs and the printoutfilm sheets on the same table, increased precision in the positioning ofthe respective portions of the photographs and film sheets is achieved.

The projected light beams of the flying spot scanners are modulated bythe photographic detail of the photographs and picked up bycorresponding photomultiplier tubes where they are converted intorelated electrical signals. These electrical signals, thoughsubstantially similar to each other in content, may be displaced inphase (relative time occurrence) depending upon the degree ofcoincidence of the terrain detail represented by the respectiveincremental areas which are being synchronously scanned. Thecorresponding electrical signals from the photomultipliers are processedby employing correlation methods in order to derive a height errorsignal from the relative phase displacement between the two signals. Inaccordance with an aspect of the invention, the height error signal isapplied as a deflection control signal to the deflection circuit of oneor both light beams so that the incremental areas scanned producerelated electrical signals having substantially zero phase displacement.The required amount of deflection control signal is also provided as adigital input to a digital computer so that it can keep track ofelevation changes. The computer then provides a signal to control thecathode ray tube employed for printing out the altitude chart.

The processing of the electrical signals is accomplished by using analogand digital techniques in combination with a small digital computer,which is used to perform the calculations involved in relating theterrain three dimensional coordinates (X, Y, Z) to the two dimensionalcoordinates of the photographs (X₁ , Y₁, X₂, Y₂) and to provide allnecessary links between the analog and digital subsystems. For example,the position of the carriage is monitored or controlled by the digitalcomputer. Information relating to the positions and orientation of thecameras at the time the aerial photographs were taken, and other relatedoptical factors, together with correcting information to compensate foraberrations in the camera lens systems and in the uniformity anddimensional stability of the photographs, is fed into the computer andis incorporated in the calculations performed therein in accordance withtransformation equations which are integrated into the computer program.

Digital techniques are employed wherever the accuracy requirement ishigher than can readily be obtained with analog devices; analog devicesare used where moderate accuracy is sufficient, and where the operationwould tax the speed limit and capacity of the digital computer.

In accordance with another aspect of the invention, part of theinformation relating the terrain coordinates to the coordinates of thephotographs is automatically provided to the computer when thephotographs are entered in the system during the initial alignmentprocedure. Thus the amount of preparation and programming necessarybefore the system may be automatically operated is materially reduced.Furthermore, the use of a combination of analog techniques to correlatethe information on the photographs forming the stereo pair and digitaltechniques to determine the magnitude of any height error and to makethe required transformations, substantially increases the speed withwhich a stereo pair can be processed. The speed of response and accuracyin positioning the selected photograph areas for scanning are furtherenhanced in accordance with an aspect of the invention by providing forcontrol of both light beams by a combination of lens servo systems andthe deflection circuitry of the light beam sources. The lens positioningservos, being electromechanical, are inherently limited in their speedof response to the control signals from the computer; however, a servoerror signal is provided to the deflection circuits to cause the lightbeams to move so as to compensate for any deviation from desiredposition of the lens system, thus achieving an effective instantaneousresponse of the beam positioning portion of the system. In accordancewith a further aspect of the invention, a stop motion circuit isprovided to deflect the scanning beam in accordance with the movement ofthe photographs for each incremental area being scanned, thuseliminating undesired relative motion between the scanning beams and thephotographs during automatic scanning. Once the profiling operation hasbeen initiated, operation is entirely automatic except for areas wherethe terrain character is not suited to automatic operation. Arrangementsare provided in accordance with the invention to vary the usualautomatic scanning procedure to compensate for such areas insofar as itis feasible. Where automatic operation is impossible, the system isarranged to provide a signal for an attendant operator who will thenassume manual control of the profiling operation until automaticoperation is again suitable.

A better understanding of the present invention may be had from thefollowing detailed description, taken in conjunction with theaccompanying drawings, wherein like reference numerals are employed torefer to like elements and in which:

FIG. 1 is a diagram helpful in explaining the principles of automaticscanning of stereoscopic photographs;

FIG. 2 represents a simplified arrangement for deriving video signalsfrom imagery detail;

FIGS. 3(a), 3(b) and 3(c) represent simplified waveforms useful inexplaining some of the principles of the present invention;

FIG. 4 is a simplified block diagram of one particular arrangement inaccordance with the invention;

FIG. 5 is a representation of an orthophoto produced by the system ofthe present invention;

FIG. 6 is a representation of an altitude chart produced by the systemof the present invention;

FIGS. 7(a) through 7(f) are representations of six different scanningpatterns provided in the system of the present inventions;

FIG. 8 is a partial block diagram of the system of the presentinvention, useful in explaining one embodiment of Height Error SensingCircuitry;

FIG. 9 is a schematic diagram of a correlator incorporated in the systemof the invention;

FIG. 10 is a partial block diagram of the system of the presentinvention, useful in explaining another embodiment of Height ErrorSensing Circuitry;

FIG. 11 is a partial block diagram, useful in explaining still anotherembodiment of Height Error Sensing Circuitry;

FIG. 12 is a detail block diagram of a particular portion of thearrangement shown in FIG. 4;

FIG. 13 is a diagrammatic perspective view of structure included in thearrangement of FIG. 4;

FIG. 14 is a table illustrating the timing and operation of the computerduring a single point loop cycle;

FIGS. 15(a) and 15(b) are block diagrams showing the arrangement ofanother embodiment of the present invention;

FIG. 16 is a partial block diagram of a particular portion of anotherembodiment of the present invention;

FIG. 17 is a combination block and schematic diagram illustrating thedetails of a portion of the arrangement shown in FIG. 15(b);

FIG. 18 is a block diagram of an arrangement included in still anotherembodiment of the present invention;

FIG. 19 is a block diagram useful in explaining the operation of aparticular portion of the arrangement shown in FIG. 15(b);

FIG. 20 is a block diagram useful in explaining a particular portion ofthe block diagram shown in FIG. 19;

FIG. 21 is a combination block and schematic diagram useful inexplaining a portion of the block diagram shown in FIG. 19;

FIGS. 22(a) and 22(b) are representations of scanning patterns useful inexplaining principles of operation of another embodiment of the presentinvention;

FIGS. 23(a) through 23(e) are representations of 5 scanning patternsuseful in explaining the principles of operation of still anotherembodiment of the present invention;

FIG. 24 is a block diagram useful in explaining the scanning patternsshown in FIG. 23;

FIG. 25 is a partial block diagram of an arrangement of anotherembodiment of the present invention; and

FIG. 26 is another partial block diagram of an arrangement of stillanother embodiment of the present invention.

Before beginning with the detailed description of the present invention,it is expedient to describe briefly, by way of introduction theprinciples involved in developing height information by automaticallyanalyzing a pair of stereoscopic aerial photographs.

Stereo pairs of aerial photographs for mapping purposes are frequentlymade using an essentially vertical aerial camera. The two pictures ofeach pair are taken at a predetermined spacing along a line of flightwhich spacing i s a compromise between maximum overlap coverage and thewide stereo base desirable for altitude measurements. The geometry of anidealized stereo system: in which the camera is exactly vertical duringtwo exposures and in which the nadir points of the photographs lie on astraight line which is substantially parallel to the line of flight, isshown in FIG. 1. It is convenient to describe the system of the presentinvention first as it would be used with such idealized photographyafter which the considerations required to adapt the system to moregeneral situations which may actually be encountered will be discussed.

In FIG. 1 a diagrammatic representation of idealized camera geometry isshown including two camera position C₁ and C₂ and an incremental groundarea generally designated by a point P with coordinates X, Y, and Z, theX coordinate being parallel to the line of flight while the Z coordinaterepresents the distance of the terrain image below the airplane. In thetwo camera positions the letter L designates the lens of a focal lengthf of the aerial camera while D₁ and D₂ represent the two photographstaken at the stations C₁ and C₂, respectively. The images of the point Pon the two photographs are indicated by P₁ and P₂ having film planecoordinates x₁, y₁, and x₂, y₂, respectively.

Assuming that in determining the elevation of the area P a height of Zhas been estimated. The system, on the basis of the estimated height,will first select and then cause the selected incremental areas havingcoordinates x₁ , y₁, and x₂, y₂ on the photographs D₁ and D₂,respectively, to be synchronously scanned, as will be described later inmore detail. Since the two synchronously scanned incremental areas at P₁and P₂ have the same images, both corresponding to the terrain area P,the two images when correlated will indicate a very high degree ofcorrelation.

However, suppose that the height of area P was estimated to be above itstrue position as at P_(e), the system will cause the photographs D₁ andD₂ to be scanned centered about the incremental areas P₃ and P₄. It isapparent from FIG. 1 that the image of the terrain area P in thephotograph D₁ namely the incremental image P₁ will appear to the rightof the center of scan about point P₃, while area P₂ representing theimage of terrain area P on D₂ will appear to the left of the center ofthe scan about point P₄, resulting in a lower degree of scanned imagerycorrelation, which in turn indicates an error in the estimated height ofthe terrain area P.

A simplified arrangement for deriving signals corresponding to errors inaltitude estimates of terrain areas represented in photographs is shownin FIG. 2, wherein scanning apparatus S₁ and S₂ (with associateddeflection circuits and lens systems not shown) direct light beams in aselected scanning pattern at predetermined incremental areas P₁ and P₂on photographs D₁ and D₂, respectively. Areas P₁ and P₂ are calculatedto represent corresponding imagery of a terrain area P having anestimated altitude Z_(e) as shown in FIG. 1. The light beams of thescanners S₁ and S₂. are modulated by the photographic details of areasP₁ and P₂ and are detected by photomultipliers 41 and 42, respectively.The outputs of the two photomultipliers are then compared to detect therelative position of corresponding imagery in the two signals. Ingeneral, the respective photomultiplier signals do not have identicalform. However, corresponding imagery in the two photographs willmodulate the scanning beams in similar fashion so that its presence maybe detected in the modulated photomultipliers' signals. For explanatorypurposes, FIG. 3(a) shows well defined peaks in the output signals ofthe photomultipliers, such as might be produced by a bright point in thefield. The peaks have zero time displacement therebetween, thusrepresenting zero error in estimating the altitude of the point of thecorresponding imagery. FIG. 3(b) shows the signals resulting from anerror in estimated altitude as diagrammed in FIG. 1 wherein the signalcorresponding to the imagery on photograph D₁ appears to the right ofthe center of the scan (assuming that scanning moves from left to right,as seen in FIG. 1), while the imagery in photograph D₂ appears to theleft of center of the scan. An error in estimated altitude of oppositedirection is represented by output signals of the photomultipliers 41and 42 as shown in FIG. 3(c).

In brief, it maybe stated that the system uses a combination of digitaland analog techniques associated with analog optical electronic scannersto change the relative positions of the scanned incremental areas withrespect to each other so as to achieve the desired time differencecorrection, and the amount of relative displacement which is needed forsuch correction is utilized to provide the desired elevation correctionindication. Most of the digital techniques are incorporated in a digitalcomputer which performs the calculations relating the terraincoordinates to the two dimensional coordinates of the aerialphotographs. The various parameters used in such calculations may betterbe under stood by referring again to FIG. 1. From the interrelationshipof corresponding elements of similar triangles, the following equationsare obtained: ##EQU1## Similar expressions for Y₁ and Y₂ can be derivedfrom a diagram viewing the system in a direction parallel to the line offlight. Since the photographs D₁ and D₂, are in line when viewed in thisdirection:. y₁ equals y₂ and the equation of proportionality can bewritten: ##EQU2##

It is obvious from the above equations that if the coordinates of P₁,P₂, x₁, y₁ and x₂, Y₂, respectively, are known for a given point, it ispossible to solve for the corresponding ground coordinates X, Y, Z. Forthe purposes of the system, it is actually more convenient to assume aposition (X, Y); this makes (x₁, y₁) and (x₂, y₂) functions of Z, whichcan be determined by identifying the points P₁ and P₂ that satisfy theabove equations (1): (2), and (3) and have correlating images. Thus,once the system is set up with a particular starting point establishedin ground coordinates, scanning can proceed on an automatic basis with Xand Y progressing in a defined manner and Z being followed by feedbackfrom the analog scanning with the corresponding values of the x₁, y₁ andx₂, y₂ relationships being determined by successive solutions of theabove equations by the digital computer. As scanning proceeds,extrapolation from altitude measurements at near by points yields aprobable altitude for the next point in line. A scan at the predictedposition then corrects the estimated value.

The idealized geometry discussed in connection with FIG. 1 is notrealized in practice. If there is any wind, the aircraft taking thephotographs will "crab" along its flight path, resulting in a "swing"error on the photographs due to the fact that the camera axis does notline up with the line of flight. In addition, flight dynamics makesachievement of a stable platform quite difficult; most photographs aremade with a certain amount of camera tilt, and it is quite possible thatthe two photographs of a stereo pair may have been taken from a slightlydifferent altitude. Problems of this type are discussed extensively in abook entitled "Manual of Photogrammetry", 2d edition 1952 published bythe American Society of Photogrammetry, and in particular in Chapter 6on "Basic Mathematics of Photogrammetry". In the operation of the systemof the invention, it is necessary to relate the ground coordinates X, Y,and Z to the film coordinates x, y. As discussed in the cited text, atpage 368ff, the coordinates of the point in the photo system may becomputed from the given coordinates of the same point in the terrainsystem through the expansion of the following matrix:

    ______________________________________                                               x            y     z                                                   ______________________________________                                        X        u.sub.1        v.sub.1                                                                             w.sub.1                                         Y        u.sub.2        v.sub.2                                                                             w.sub.2                                         Z        u.sub.3        v.sub.3                                                                             w.sub.3                                         ______________________________________                                    

The various parameters u, v, w are the direction cosines of therespective photograph system coordinate axes in the ground system ofcoordinates and may be further defined as set forth in the tabledesignated (10.26) on page 368 of the above-mentioned text astrigonometric functions of tilt, swing and azimuth. The trigonometricterms are all constants for a particular photograph so that theequations become simple, once the appropriate numerical values areintroduced. In accordance with the mathematical development above, thedigital computer of the system of the invention is programmed tocalculate photograph positions x, y corresponding to particularfunctions of the ground positions X, Y, Z. These may be considered asthe transformation functions of the system and in general form may berepresented as

    x=G(X, Y, Z)                                               (4)

    y=H(X, Y, Z)tm (5)

It will be understood that the equations (4) and (5) are unique for eachphotograph so that the computer must solve the two equations twice, oncefor each photograph. The machine coordinates are appropriate scalings ofthe plane terrain coordinates X, Y. As will be explained in furtherdetail, the computer develops the value of the height coordinate Zduring the operation of the system.

It should be mentioned that in addition to the above trignometricfunctions, which are related to the attitudes of the cameras as thephotographs are taken and which are included in the coefficients of theparticular transformation functions as represented by equations (4) and(5), the coefficients are modified in accordance with the error involvedin placing the photographs on the compilation marriage, in order thatthe appropriate transformation may be properly related to the actualposition of the photographs in the system. This modification isperformed at the time of setting up the computer for a given set ofphotographs preparatory to automatic operation of the system, and thecorresponding information is automatically fed into the computer duringthe manual setup procedure.

A simplified block diagram of one particular embodiment of the presentinvention is shown in FIG. 4, in which an automatic map compilationsystem is shown including a movable compilation table 40 on which thephotographs D₁ and D₂ are positioned, together with an orthophoto filmsheet N₁ and an altitude chart film sheet N₂. Both photographs arescanned and the resulting signals analyzed to determine the altitude ofthe corresponding terrain. The orthophoto film sheet N₁ is provided forproducing an orthographic photo map, and the film sheet N₂ is providedfor the production of an altitude chart. A mechanical drive mechanism 44is connected to the table 40 in order to move it in a precise profilingpattern in both the x and y directions. The profiling pattern, asdistinguished from scanning, proceeds in alternating y directions withstep-over in the x direction by a predetermined incremental distance atthe end of each y traverse. A pair of photomultiplier tubes 41 and 42,associated with the respective photographs D₁ and D₂, are affixed nearthe table 40 so as to be in position to collect the light beams asmodulated by passing through the photographs.

The x and y positions of the compilation table 40 are reported to acomputer 50 by means of a position readout stage 45. From theinformation thus reported, the computer further controls the relativepositions of lenses 46 and 47, by means of x servos 48 and 49 and yservos 52 and 53 through digital-to-analog (D/A) converters 56, 57, 58and 59, respectively. In this fashion the light beams are centered onthe appropriate computer commanded incremental areas of the respectivephotographs for scanning. The light sources for scanning the respectivephotographs are a pair of flying spot scanners 61 and or similar lightsources easily adaptable to electronic scanning and switchingtechniques. The flying spot scanners provide intense, small spots oflight of very short persistence that move back and forth in a TV-likescan, with the fast scan in the x direction on the respectivephotographs, to convert the photographic imagery to electrical signals,Thus, scanning proceeds basically over incremental line elements in thex direction while profiling occurs with the movement of the table 40 inthe y direction. Cathode ray tubes 63 and 64 are provided for printingout the respective orthophoto and the altitude chart on the two filmsheets N₁ and N₂. Deflection circuits for the flying spot scanner 61, 62and for the cathode ray tubes 63, 64 are designated 66, 67, 68 and 69,respectively. Each of the deflection circuits 66-68, together with othercircuitry described hereafter, causes the associated tube to generate anappropriate raster, which may be defined as a predetermined pattern ofscanning providing substantially uniform coverage a predetermined area.The deflection circuit 69 is arranged to maintain the image of theraster formed by the electron beam of the tube 64 in a substantiallyfixed position on the photograph during a height evaluation. A syncpulse generator 71 for generating x and y sync pulses necessary for thescanning operation and the print out of the orthophoto film sheet: iscoupled to each of the deflection circuits 66-68.

The computer 50 and a computer terrain tilt simulator 72, which may beregarded as a special subsystem, control a raster shape modulator 75which generates sweep signals and supplies them to the deflectioncircuits 66 and 67 of the flying spot scanners. The signals from theraster shape modulator adjust the shape and size of the scanning rastersof the two scanners so as to compensate for terrain tilt and altitudevariations. The output signals from the photomultipliers 41 and 42,which correspond to the light beams of scanners 61 and 62 modulated bythe image detail in the scanned portions of photographs D₁ and D₂, areapplied to a height error sensing circuit 76. The error sensing circuit76 employs correlation techniques to measure altitude errors andsupplies signals to the computer 50 through counter 77, so thatappropriate height corrections may be made in the previously estimatedaltitude value of the particular area which is being scanned. The errorsensing circuit 76 also supplies through counter 77 and a converter 78 aheight error signal to the deflection circuits 66 and 67 causing arelative displacement between the centers of the two scanning rastersthereby varying the incremental areas on 30 the photographs which aresynchronously scanned, until a degree of signal coincidence similar tothat shown in FIG. 3(a) (indicating zero height error) is approached.The output signals from the photomultipliers 41 and 42 are also suppliedto a selector circuit 81, which selects the signal from one of thephotomultipliers and supplies it to the orthophoto tube 63 for exposingfilm sheet N_(I) so as to print an orthographic projection of theincremental area scanned thereon. Such selection may be manuallyaccomplished by selecting either of the signals, or electronictechniques may be employed to select the better of the two video signalsfrom the two photomultipliers.

As previously explained the output film sheets N₁ and N₂ are mounted onthe compilation table 40 so as to follow the motion of the photographsD₁ and D₂, and be exposed in accordance with information derived duringthe scanning operation of the system. The cathode ray tube 63,associated with the orthophoto film sheet N₁, is controlled to reproducein a TV-like manner terrain character information received from aselected photomultiplier 41 or 42 while the cathode ray tube 64,associated with the altitude chart film sheet N₂, is controlled by thecomputer 50 to produce height information. These two cathode ray tubesprovide the means by which the output data is printed out on thephotosensitive sheets, thereby producing an orthophoto and an altitudechart. The orthophoto cathode ray tube 63 receives its signal from theselector 81 as an input to its intensity control grid so that theelectron beam of the tube 63 is appropriately modulated with the terraininformation. The imaging optics, represented by the lens 83, focusesthis image on the sensitized film sheet N₁ . An orthophoto is exposed onthe film as the compilation table 40 moves in a profiling pattern aboutthe stereo field, causing diapositives D₁ and D₂ to be incrementallyscanned by flying spot scanners 61 and 62 and the image detailtransferred to appropriate portions on the film sheet N₁. The othercathode ray tube 64 receives its intensity grid modulating signal fromthe computer 50 during the automatic scanning 30 mode of operation. Thecathode ray tube 64 through a focusing lens 84 traces out only a line onthe altitude chart film sheet N₂ as the compilation table 40 proceeds inits y profiling direction.

A typical orthophoto is represented in FIG. 5. As shown, the orthophotois printed, profile by profile, as the film sheet N₁ is moved back andforth under the cathode ray tube 63 to reproduce the detail appearing inthe aerial photographs. FIG. 6 represents a corresponding altitude chartprinted out on the film sheet N₂ by the cathode ray tube 64. It will benoted that the altitude chart is printed in three distinct shades, inthis case black, gray and white. Each shade indicates a differentelevation range as selected by the operator in setting the scale of themap (for example. 1240-1260 feet) and the three shades follow inrepetitive sequence for a monotonic change of elevation. By using thismanner of displaying elevation information, the direction of change ofelevation is indicated in addition to the fact that a change occurs at aparticular point. Thus, a change from black to gray may mean that theelevation has changed from the range of 1240-1260 feet to the nextlower: range, 1220-1240 feet), whereas a change from the same blackregion to white would indicate that the elevation has changed from therange 1240-1260 feet to a higher range above 1260 feet. It is thus asimple matter to determine the usual contour lines by connecting therespective corresponding points of changes in elevation, as indicated onthe altitude chart of FIG. 6.

The manner in which scanning of a particular incremental area proceedsmay be described by referring to FIG. 7 comprising patterns 7(a)-7(f).In FIG. 7(a), the scanning pattern for one flying spot scanner isrepresented. The electrical signals produced by each of thephotomultipliers 41 and 42 during a single complete scan are suitablygated so as to develop the five effective scan patterns which arerespectively designated 7(b), 7(c), 7(d), 7(e) and 7(f). Correspondingsignals from each of the two photomultipliers 41 and 42 for each of therespective scan segments are correlated in order to produce height errorsignals for each segment. The central area signal, corresponding to thescan segment 7(b), develops the height error signal for application tothe computer 50 in order to correct previously approximated elevationvalues for the particular scanned areas. The left and right segmentsignals, corresponding to the patterns 7(c) and 7(d), appropriatelysummed may yield an x-axis tilt error signal, while the top and bottomsegment signals, corresponding to patterns 7(e) and 7(f), appropriatelysummed may yield a y-axis tilt error signal. When correlation betweenrespective signals is poor over the central area, the signals from allfive segments of the patterns shown in FIGS. 7(b)-7(f) can be combinedto provide a more satisfactory height error signal. The flit errorsignals may be utilized in the raster shape modulator 75 which aspreviously explained skews the shapes of the scanning rasters so as toaccount for slope variations of the areas under investigation.Typically, the raster will take the form of a parallelogram (FIG. 7(c))obtained by a proper summation of fast and slow sweep components appliedto the sweep axes of the scanning circuitry. The signals may also be fedto the computer 50 to permit accurate extrapolation of the next altitudevalue:, thus substantially reducing the height error requiringcorrection in the next measurement cycle.

Tilt error signals may also be derived from the computer wherein terraintilt components may be computed on the basis of elevation variations ofneighboring points previously calculated in the compilation operation.

In one embodiment of the present invention a correlator circuit 90 asshown in FIG. 8 is employed in the error sensing circuit 76 (FIG. 4) forproviding height error signals by correlating signals corresponding toscanning patterns similar to FIG. 7(b). The output signals of thephotomultipliers 41 and 42, which are functions of the intensities ofthe light beams as modulated by the photographs D₁ and D₂, are used asinput signals to video amplifiers 85 and 86 which are respectivelyconnected to automatic gain controlled amplifiers (AGC) 87 and 88. Theoutputs of the amplifiers energize a pair of correlators 91 and 92through delay stages 93 and 94. For each correlator 91 or 92, one inputis applied directly from one photomultiplier (through a correspondingvideo amplifier and an automatic gain controlled amplifier) while theother input is applied from the other photomultiplier through acorresponding delay stage 93 or 94. An input error signal derived from adifference amplifier 95 energized by the outputs of the correlators 91and 92, by comparing the respective outputs of the correlators againsteach other so that the resultant signal is a measure of the discrepancybetween the two correlator outputs. Because of the delay interposed inone of the inputs by the delay stage 93 or 94, the correlation betweenthe delayed signal and the undelayed signal is either improved or madeworse, depending upon whether or not the time displacement of thephotomultiplier signals is compensated by the delay. The time displacedsignals, such as those shown in simplified form in FIGS. 3(b) and 3(c),are thus brought into better correlation in one of the correlators whilethe correlation is diminished in the other correlator. As a result, theoutput of one correlator, such as 91, increases while the output of theother correlator, such as 92, decreases. The difference between the twoas derived in the difference amplifier 95 provides an error signal of aparticular polarity and having a magnitude indicative of height error.If on the other hand the signals are coincident in time, indicating theestimated altitude to be correct, the correlation in each of thecorrelators 91, 92 is off by the amount of delay introduced at the inputthereof. The outputs of the correlators 91 and 92 are thus the same sothat the resultant error signal, equal to their difference, is zero. Asexplained in connection with FIGS. 3(a) through 3(c), the height errorsignal is thereby a function of the magnitude and direction of deviationfrom true elevation. The output of the correlator circuitry 90 issupplied to an integrator 99 of an analog error detector designatedgenerally by numeral 101. The output of the integrator 99 is applied toeach of two Schmitt trigger circuits 102 and 103, one intended to sensepositive error signals and the other to sense negative error signals. Asis well known, the Schmitt trigger circuit changes state when the inputsignal passes across a predetermined threshold level. As the cumulativeheight error, integrated in the integrator 99 from the correlatoroutput, reaches this predetermined level, the appropriate Schmitttrigger is energized, causing a pulse to be applied to the reversiblebinary counter 77, which serves as an analog-to-digital converter. Atthe same time the output of the energized Schmitt trigger serves toreset the integrator 99. The output of the reversible counter 77 isconnected through a digital-to-analog converter 78 to the deflectioncircuits 66 and 67 in order to relatively displace the scannedincremental areas so that after a number of successive steps thecorrelated signals are made to be coincident in time. The reversiblecounter is also connected to the computer 50, which at a predeterminedpoint in the programming cycle reads the counter as an error inestimated height of the scanned area, and makes appropriatemodifications of the altitude value stored therein. The computer alsoresets both the counter 77 and the integrator 99 so that height errorevaluations of the next incremental areas to be scanned may proceedindependently of the last measurement.

A schematic diagram of a correlator such as may be used in correlators91 and 92, is shown in FIG. 9. This correlator is of the multiplyingtype and comprises a plurality of resistors 105, diodes 106, and acapacitor 107 interconnected across the secondary windings of pair oftransformers 108 and 109, arranged to receive as inputs the signals Aand B which are to be correlated. The diodes 106 have their polaritiesas shown with adjacent pairs of diodes poled in opposite directions. Thediodes 106, together with the resistance 105, provide an approximationto a square law voltage-current relationship. The push-pull transformers108 and 109 are used to make available both polarities of the two inputsignals, A and B. The output voltage e_(o) across the capacitor 107 maybe written: ##EQU3## where K is an arbitrary constant ofproportionality, R is the resistance of one of the resistors 105 and Cis the capacitance of the capacitor 107. Depending on whether the sumsof A+B and A-B are positive or negative, different ones of the diodes106 will be rendered conducting in order to transfer charge to or fromthe capacitor 107. The above equation simplifies to: ##EQU4## so thatthe output voltage e_(o) is a measure of the average product of the twosignals A and B, and; hence, indicates the degree of correlation betweenthem. For example, if A and B are signals having random characteristics,the product will have many positive and negative contributions and,hence, a low average; whereas if A and B are identical, theinstantaneous products are always positive and, hence, form anon-cancelling sequence. The desired integral represented by equation(7) is only approximated with the simple circuits shown in FIG. 9, butthe depicted circuit is quite effective as a means for signalcorrelation.

Referring again to FIG. 8 wherein height sensing circuitry is shown, itseems apparent that the complete frequency spectrums of the signals fromphotomultipliers 41 and 42 are used in correlator circuit 90 forderiving the height error signals, thereby taking advantage of all theinformation in the imagery in the two photographs. However, experiencehas shown that in some cases it may be desirable to derive the heighterror signal from a combination of signals representing predetermineddegrees of image detail. Such combinations permit very close followingof the terrain altitude variations over relatively smooth surfaces witha minimum tendency of the system to "get lost" in terrain of greatlyvarying altitudes.

Therefore, in another embodiment of the present invention a heightsensing circuit as shown in FIG. 10 is incorporated wherein differentimagery spectrums are combined. The correlator circuit 90 shown therein,which is energized by signals from amplifiers 87 and 88 operates in amanner identical to the one described in connection with FIG. 9.However, as shown in FIG. 10, the outputs of video amplifiers 85 and 86are also supplied to a filter unit 111 wherein predetermined highfrequencies are filtered out before supplying the signals to automaticgain controlled amplifiers 87' and 88' which energize a correlatorcircuit 90' identical in function and performance to correlator circuit90. The outputs of correlator circuits 90 and 90' are then supplied to asumming integrator 99a similar to integrator 99 of FIG. 8 except thatsumming integrator 99a integrates the combined input signals fromcorrelator circuits 90 and 90'. The rest of the height sensing circuitryis identical to the circuitry shown in FIG. 8 and therefore itsdescription will not be repeated.

In the above described embodiments of the height sensing circuitry(FIGS. 8 and 10), the digital height error signal from the counter 77 isconverted in the D/A converter 78 to an analog signal used to relativelydisplace the rasters of the scanners 61 and 62 (FIG. 4) by supplyingappropriate signals to their respective deflection circuits 66 and 67.As explained herebefore, by relatively displacing the rasters withrespect to one another, the positions of the corresponding imagery inthe two scans are made to vary, so that homologous areas on bothphotographs are synchronously scanned. The same desired result may beachieved by differentially delaying the output signals of thephotomultipliers 41 and 42 so that the apparent positions of thecorresponding imagery in the two scans seems to be the same. Themagnitude of such differential delay would then indicate the truepositions of the imageries within their scans which is the desiredindication of the error in estimated height.

FIG. 11 partially represents a height sensing circuit wherein suchdifferential delay techniques are incorporated. The outputs of videoamplifiers 85 and 86 energize a differential delay unit 112. The unitmay be described as comprising independent delay lines which, dependingupon the instructions from the delay selection logic lines, may insert adelay in one line or the other depending on the polarity of the count inthe counter 77, with the magnitude of the delay being dependent on themagnitude of the count in the counter 77. The remainder of the heightsensing circuitry is identical to the circuitry shown in FIG. 10.

As previously stated, the photographs D₁ and D₂ are scanned in a patterndefined by the shape of the scanning raster which is controlled by theraster shape modulator 75, wherein the slope components of the scannedterrain are accounted for. The need to modify the shape of the rastersof the flying spot scanners 61 and 62 in order to compensate for theterrain's slope components may be better understood from a considerationof the above-mentioned transformation functions (4) and (5) which arereproduced below for convenience:

    x=G(X, Y, z)                                               (4)

    y=H(X, Y, Z)tm (5)

The transformation functions relate the Cartesian coordinates of theflying spot scanner rasters as they appear on the respective photographsto the ground coordinates X, Y, Z.

Let it be assumed that the computer is considering a point on theterrain having coordinates X, Y, Z, and the scan on the first photographwill be centered at (x, y) as given by the functions G, H defined inequations (4) and (5). As the scan on the orthophoto: moves from X, Y toX+ΔX, Y+ΔY, the scan on the photographs should be moved accordingly sothat the instantaneous point under observation agrees with the pointbeing printed out by the orthophoto tube. The relationship of theincremental scans may be defined by

    Δx=G.sub.x ΔX+G.sub.y ΔY+G.sub.Z ΔZ(8)

However, ΔZ is a function of height error related to ΔX and ΔY asmodified by the terrain slope and may be represented as

    ΔZ=Z.sub.X ΔX+Z.sub.Y ΔY                 (9)

combining equations (8) and (9) yields:

    Δx=[G.sub.X +G.sub.Z xZ.sub.X ]ΔX+[G.sub.Y +G.sub.Z Z.sub.Y ΔY                                                  (10)

The function (5) may similarly be expanded yielding

    Δy=[H.sub.X +H.sub.Z Z.sub.X ]ΔX+[H.sub.Y +H.sub.Z Z.sub.Y ]ΔY                                                 (11)

The functions corresponding to the second photograph may be expanded ina similar manner.

From a careful observation of the functions (10) and (11), it is seenthat both the x and y scan components of the photograph scanner includeboth fast (ΔX) and slow (ΔY) scanning components of the orthophoto.Further, the photograph scanning pattern is a function Z_(X) and Z_(y)which are the terrain slope components in the direction of the line offlight and in a perpendicular direction thereto, resulting in thephotographs being scanned by a skewed raster pattern so that theinstantaneous area under observation corresponds to the area printed outon the orthophoto film sheet N₁. As the height Z varies with the groundcoordinates X; Y, the coefficients of the respective X and Y variableschange accordingly, thus adjusting for the scaling and shaping of theflying spot scanner rasters in order to match the orthophoto scan forthe different terrain involved. With vertical photography and levelterrain, the shape of the photograph raster is identical with theorthophoto raster. When the terrain slopes, however, the photographraster becomes parallelogram-shaped to match the square orthophotoraster:.

As previously noted, the skewing of the scanning raster improves thecorrelation and the quality of the orthophoto. However in somesituations the results may be adequate without incorporation of thisfunction.

FIG. 12 represents one embodiment of a raster shape modulator arrangedto correct for variations in altitude and for slopes in the direction ofthe scan for essentially vertical photography. This configuration wouldbe used with a raster that is wide in the x direction thus providingmore time in each line scan for correlation but narrow in theperpendicular (y) direction to simplify the shaping requirements Itwould therefore permit a wide separation between successive profilesthus speeding up the compilation process. The extension to a rastershape modulator that corrects for slopes in both directions andtherefore permits still greater compilation speeds follows in an obviousmanner from the simplified version.

The raster shape modulator shown is implemented in accordance withequations (10) and (11) wherein Z is arbitrarily made equal to zero,i.e.

    Δx=(G.sub.X +G.sub.Z Z.sub.X) ΔX+G.sub.Y ΔY(12)

and

    66 Y=H.sub.X ΔX+(H.sub.Y +H.sub.Z Z.sub.Y) ΔY  (13)

For vertical photography where ##EQU5## as derived from equations (1)and (3). The scan equations (12) and (13) become ##EQU6##

FIG. 12 shows a Z register 115 obtaining the local altitude from thecomputer 50. This operates through a D/A converter 116 to perform theoperation f/Z wherein f is applied as a fixed voltage input to D/Aconverter 116. The output of the D/A converter 116 is applied to asecond D/A converter 118 which also operates from the Z register 115 sothat its output is f/Z². This output is applied to a third D/A converter119 which operates from a register 120 holding the X-axis slopecomponent Z_(X) supplied thereto from the computer 50. The output of theD/A converter 119 is therefore fZ_(X) /Z² and its complementary output-fZ_(X) /Z₂ is also made available. These two outputs are applied acrossa potentiometer 122 linked to the X motion of the carriage 40 so that itmultiplies the input by X providing the term -fZ_(X) X/Z². The two termsf/Z and -fZ_(X) X/Z² from the D/A converter 116 and the potentiometer122 respectively are appropriately summed in a fast integrator 125 whoseoutput is made zero at the line sync time supplied thereto from the syncpulse generator 71 (FIG. 4), The output of the fast integrator 125provides the function Δx indicated by equation (16) and is supplied tothe X-axis of the deflection circuits 66 and 67 (FIG. 4) . The twovoltages +fZ_(X) /Z² from the D/A converter 119 are also supplied to apotentiometer 123 linked to the Y motion of the movable table 40 whichthen provides the output -fZ_(X) Y/Z² ; this signal is applied to a fastintegrator 126 whose output is made zero at the line sync time suppliedthereto from the sync pulse generator 71. The output of the D/Aconverter 116 which is f/Z is supplied to a slow integrator 128 which issynchronized by a frame sync pulse from the sync pulse generator 71, sothat the output of slow integrator 128 equals f/ZΔY. The outputs fromthe fast integrator 126 and the slow integrator 128 are summed up in anadder circuit 129, the output thereof being the function Ay indicated byequation (17) and supplied to the Y-axis of the deflection circuits 66and 67.

The flying spot scanner driving circuitry, comprising the deflectioncircuits 66, 67, the sync pulse generator 71 and the raster shapemodulator 75 (FIG. 4), generates one of three rasters for use at eachflying spot scanner. A three-inch by three-inch raster is used toprovide a relatively large scanned area for a stereo viewer 82(resulting in an area of 200 mils by 200 mils on the photographs) foruse during a manual setup procedure. The 1.5-inch by 1.5-inch rasteralso is useful during the setup operation in that it provides a smallerarea (100 mils by 100 mils) scan and hence a magnified image on thestereo viewer 82. The actual size of the image observed is the same forboth raster configurations. In the automatic mode, a nominal 3/4-inch by3/4-inch, 40-line, 120 cycle raster is generated by the electronic sweepcircuitry. This raster size results in a scanned area of 50 mils by 50mils on each photograph. The geometrical configuration of the rasterwhen in the automatic mode is determined by the raster shape modulator75. The light beams from the flying spot scanners 61 and 62, asmodulated by the associated photographs D₁ and D₂, are sensed by thephotomultipliers 41 and 42. The resulting electrical signals are used torecreate the imagery for the orthophoto production, for direct viewingat the stereo viewer, and for use in the height sensing circuitry.

In one embodiment of the present invention, the compilation table 40 ismounted as shown in FIG. 13 on a pair of carriages 131 and 132controlled to provide the desired traverse motion of the table 40. The Ycarriage 131 is mounted on roller bearing blocks riding on an inverted"V" groove-way which serves to support the relatively heavy Y carriagewith a minimum of undesirable sideplay. The Y carriage is driven by ahigh precision ball-nut lead screw 134 using preloading in the ball-nutto eliminate back lash. A 400 cycle, two-phase servomotor 135 is coupledto supply driving power for the Y carriage 131. With this arrangement,the Y carriage 131 moves smoothly with its position reported to thecomputer 50 by a two-bit gray code shaft-to-digital encoder 136.

In the traverse movement of the compilation table 40, it is expedient tomove the X carriage 132 in a precision step-by-step manner, therebyminimizing the problem of reporting its position to the computer 50. TheX carriage 132 is supported by ball-bushings riding on hollow shaftswhich are mounted on the Y carriage 131. The X carriage 132 is connectedto a drive servomotor (not shown) through a ball-nut lead screw 88 andantiback-lash gears. A stepping motor is supplied to drive the Xcarriage 132 in increments of 0.01 inch. When more than one step isrequired, the motor continues to step until the desired position isreached. The number of steps taken is counted by the computer 50,thereby making the X carriage position known to the computer.

In the foregoing arrangement, the control of the position of thecompilation table 40 is arranged to achieve satisfactory positioningaccuracy. Control of the stepped position of the X carriage is accurateto a deviation of approximately 0.0002 inch. Despite the weight of therelatively heavy Y carriage, the control system is capable of stoppingthe carriage at the end of each profile traverse from a velocity of oneinch per second in sixty milliseconds with a deceleration distance ofapproximately 0.035 inch. Since the position of the Y carriage isreported to the computer in digital code: and since the position of theX carriage is fixed with such precise accuracy, the system of theinvention is able to make measurements at predetermined points which arewithin very close tolerances of the computer-defined values.

In order to produce the required relative displacement between theincremental areas selected for scanning on the respective photographs,the lens systems associated with the flying spot scanners 61 and 62 maybe arranged to be movable under the control of the computer 50. The twolens systems are identical and provide a 16:1 reduction in image size ofthe photographs. Each system comprises a two axis, x and y, servo. Agear reduction drive is utilized including preloaded ball-nut leadscrews to eliminate back/ash and provide precise positional control ofthe lenses. The orthogonality of the x and y axes and low side play oneach axis are insured by using preloaded ball bearings rolling inV-groove-ways similar to the support mechanism for the compilation tableY carriage. The control circuitry consists of closed loop 400-cyclepositioning servos, such as the servo 48 (FIG. 4), which are controlledby the computer through digital-to-analog converters. Precisionpotentiometers are used as position feed-back transducers to completethe feed-back loop.

In order to improve the accuracy and speed of response of the system,servo error signals may be applied to the deflection circuits 66 and 67of the associated flying spot scanners 61 and 62 from the servo units 48and 52, 49 and 53, respectively, in order to compensate for anyinstantaneous lens servo position error and especially for the servoinertia which limits the speed of response of the servo system toenergizing signals. This error signal is arranged to deflect the rasterof the flying spot scanner so that the scan will center on the desiredarea of the photograph, even though the lens is still being moved intoposition by its positioning servo. This arrangement produces a very fasteffective response for the lens positioning system so that the automaticoperation of the over-all system may proceed rapidly with highprecision.

The system, according to one embodiment of the invention, examinesgeographical positions which are spaced in multiples of 0.01 inch at theorthophoto scale in the Y traverse direction, which is thusperpendicular to the flight line on the orthophoto. The correspondingpositions on the two photographs will be spaced a like amount if theincremental area being scanned is at the selected reference altitude.The spacing may be greater for areas above reference altitude and lessfor areas below reference altitude because of corresponding changes inscale on the photographs.

The scanning operation is controlled in accordance with the broadoutline for a computer program which is diagrammed in FIG. 14. Theoutline represented in FIG. 14 may be referred to as a point loop cycle,and represents a preset control sequence for the computer 50 during theanalysis of an individual incremental area for one set of homologouspoints on the respective photographs. The cycle requires just over 15milliseconds to complete for the particular computer 50 which isutilized in the described arrangement of the invention. During the pointloop cycle, the gray code information derived from the Y positionreadout stage and the elevation change information derived from theerror sensing stage are supplied to the computer. Utilizing informationalready stored in the computer from previously scanned homologouspoints, a new height is estimated for the next position to be scanned,the new photograph coordinates are calculated and the correspondinginformation is applied from the computer output to the appropriateportions of the system to achieve the desired centering on thephotographs for the proper analysis of the new homologous points. Thecycle is then completed and is repeated over and over again during theautomatic scanning procedure, perhaps as many a 500,000 times for oneset of photographs.

In the operation of the system of the invention as represented in FIG.4, the automatic scanning of the photographs, the analysis of thederived information, and the printout of the orthophoto and the altitudechart proceed under the over-all control of the computer 50. Althoughany large fast computer with the proper input-output characteristics maysuffice for this function, the system has been arranged to operate witha relatively inexpensive, digital computer since the complexity of thecalculations to be performed therein is relatively low. When the systemis being operated in the automatic mode of operation, the digitalcomputer 50 is operated in accordance with a specified program, tocalculate where the homologous points corresponding to a givengeographical location and estimated altitude are to be found on the twodiapositives. The various servos for driving the lenses 46, 47 arecontrolled by the computer output to move the electronic scanning to theareas on the diapositives which are calculated to be centered on thehomologous points. Associated correlation circuitry in the error sensingcircuit 76 then evaluates the relative displacement of the scanned areasand provides signals which are fed back to the computer to indicate theerror in the estimated altitude. The error signal and resulting altitudeare then used for making the next altitude estimate. The operationproceeds with the computer making the calculation for the next point inthe sequence. As the operation progresses, the terrain character isprinted out on the orthophoto film sheet N₁ and the altitude data isprinted ont on the altitude chart film sheet N₂.

It is apparent from the above description of the operation of thepresent invention that the accuracy of the output information printedout on the two film sheets directly depends on the computer'scalculations, the ability of the computer to position the analog systemso that preselected incremental areas may be synchronously scanned, andthe accuracy of correlating the modulated scanning signals.

One embodiment of the system incorporating some particular aspects ofthe present invention which greatly increase the over-all accuracy ofthe system may better be described by reference to a more detailed blockdiagram of the arrangement of FIG. 4, as is presented in FIGS. 15(a) and15(b). In this more detailed block diagram, corresponding individualelements are designated by like reference numerals. The computer 50 isshown coupled to receive input information in the form of programmedinstructions from input unit 140, presented, for example, on punchedpaper tape automatically read by a tape reader 141. The unit 140 furthercomprises a data writer 142, which may be in the form of a speciallyconnected typewriter, used by the operator for special purposes in thevarious modes of the operation of the system. For example, in preparingthe system for automatic scanning, a mode selector switch 145 comprisinga plurality of movable contacts 145A-145F is first placed in the manualposition. The manual mode of operation is provided for determiningcertain constants to be supplied to the computer for successfuloperation in the automatic mode. Alignment in the manual mode involves afairly precise positioning of the photographs D₁, D₂ with reference tothe computer controlled positioning mechanism for the compilation table40. It will be understood that the respective photographs D₁ and D₂ willhave particular points, referred to as passpoints, identified in eachphotograph with the photo and ground coordinates of the respectivepasspoints already determined and fed into the computer 50 from the tapereader 141. In order that the system may be prepared for operation inthe automatic mode, it is necessary to provide certain photogrammetricparameters to the computer 50 by means of the data writer 142 and thepaper tape 141. These parameters relate to the Cartesian coordinatepositions of the cameras taking the respective photographs and of therespective passpoints on the photographs, to the correction of cameralens error, and to the elevation interval code information. After thephotographs D₁ and D₂ are placed in position on the compilation table40, the X and Y carriages and the associated lens systems are moved bythe operator under the control of the computer 50 to the first selectedpasspoint. When the carriages have reached the proper location, thepasspoint may be observed on a stereo viewer 82 which includes a displaycathode ray tube and an accurately positioned electronically generatedcross-hair for each photograph. The stereo viewer 82 is provided withduplicate magnified views of the respective incremental photograph areasincluding the particular passpoint being scanned, when the system isoperating in the manual mode. By controlling the computer 50 via thedata writer 142, the operator causes the system to properly align thefirst passpoint on each diapositive with the cross-hairs of the stereoviewer 82.

During this phase of the alignment process, the photo coordinates of thefirst passpoint previously fed into the computer 50 from tape reader 141remain stored therein while additional coordinate information related tothe first passpoint, as properly aligned with cross-hairs of the stereoviews, is supplied to the computer by the compilation table positionindicators. The operator then actuates the computer through data writer142 to move the photographs to the position for scanning the secondpasspoint. The alignment process is then repeated so that the secondpasspoint becomes centered on the corresponding cross-hairs in thestereo viewer 82. The computer then receives additional coordinateinformation related to the actual position of the second passpoint inthe system. On the basis of all the coordinate information relating toboth passpoints as initially supplied to the computer by tape reader 141and during the alignment process of both points, the computer updatesand calculates new coefficients for the transformation equations of thecomputer program, which accurately relate the particular points in thefield of the photographs to the machine coordinates of the compilationtable 40. The operator then directs the system through the computer toscan a third passpoint in order to verify that the new coefficients ofthe transformation equation were accurately calculated. This isaccomplished by observing the third passpoint on the photographs withreference to the cross-hairs of the stereo viewer 82. With thesuccessful location of the third passpoint on each photograph, thesystem is placed in the automatic mode by properly switching the modeselector switch 145.

The alignment process described above greatly increases the accuracy ofthe system by eliminating the need for precisely adjusting the physicalpositions of the photographs on the compilation table, and by furtherupdating all coordinate information after the photographs have actuallybeen fixed within the system. The alignment process further serves tocompensate for any drift in the electronic circuitry which may haveoccurred, thus adding to the stability and accuracy of the system.

The mechanical coupling provided by the compilation table 40 between thephotographs D₁ and D₂ and the printout negatives N₁ and N₂ isrepresented in FIG. 15(a) by the dashed line 100 also extending to theshaft encoder 136, which provides the desired Y position of the table40, and to various potentiometers 122, and 123 which generate signalscorresponding to the position of the table 40 for application to theraster shape modulator 75.

During the time that the system is being controlled in the manual mode,neither the orthophoto cathode ray tube 63 nor the altitude chartcathode ray tube 64 is actuated to print out orthographic or altitudedetail on the film sheets N₁ or N₂ respectively. However, while thesystem is positioned to scan the various passpoints during the alignmentprocedure through operation in the mark mode, it may be desirable totransfer the passpoint positions and other particular marks to theorthophoto and the altitude chart. This may be done on a selective basisby switching the mode selector switch 145 from the manual to the markmode position and by energizing character printing circuitry comprisinga character generator 146 and a character selector switch 147. Whenenergized, the character generator 146 provides appropriate characterprinting signals to the deflection circuitry of the printout cathode raytubes 63 and 64, These signals are 400 cycle voltages arranged to haveappropriate amplitude and phase relationships so that a desiredcharacter in the form of a selected Lissajous figure is constructed. Thecharacter selector switch 147 determines which of the Lissajous figuresis developed at the cathode ray tubes 63, 64. For example, to generate acircle, two 400 cycle voltages 90° out of phase are applied from thecharacter generator 146 to the orthogonal deflection plates of thecathode ray tubes 63, 64 via the deflection circuits 68, 69. From thestandpoint of system operation, the exact time in the operating cyclefor printing the selected characters is not important. However, it ispractical to perform the character printing operation during the manualmode of operation.

Once the appropriate information is supplied to the computer 50,including that which is developed during the alignment procedure justdescribed, the map compilation system of the invention is ready for theautomatic profiling operation wherein the system examines geographicpositions which are spaced by multiples of 0.01 inch at the system scalealong a profile perpendicular to the flight line. These positions aresupplied by the system 44 and shaft encoder 136 to the computer 50 whichthen calculates the location of the homologous areas on the twophotographs corresponding to the geographic location. The computer 50,after performing the necessary calculations, commands, through a switch150, the lens servos 48, 49, 52 and 53 to move lenses 46 and 47 so thatthe homologous incremental areas corresponding to the given geographiclocation may be synchronously scanned in order to automatically obtainthe true height value of the terrain location. The exact position of thetable is available to the computer 50 through the X steps as they occurand through the shaft encoder 136 which monitors the Y table position.The computer then compares the actual and desired positions of the tableand if any difference in the values is detected, digital signals aresupplied by the computers to servos 48, 49, 52 and 83 through thedigital-to-analog converters 56-59 controlled by a D/A control unit 151.The respective servos which are coupled to the focusing lenses 46 and 47of the scanners 61 and 62 shift the lenses 46 and 47 by appropriateamounts so that the desired incremental areas as initially determined bythe computer 50 are scanned. To further increase the speed and accuracyin selecting the appropriate areas for scanning the outputs of the xservo 48 and y servo 52 are connected to the deflection circuit 66 sothat any delay in positioning the lens 46 by the lens servos 48 and 52is compensated by applying a signal proportional to the servo error tothe centering control of the flying spot scanner 61. This signaldeflects the raster, thereby compensating for the servo position errorso that the desired incremental area on the photograph D₁ is scannedwith a minimum of time lag. Similarly, the servos 49 and 53 are coupledto the deflection circuit 67 to deflect the raster of flying spotscanner 62 so that the proper area is scanned on the photograph D₂.

Once the appropriate areas are positioned under their respectivescanners, the light beams of the flying spot scanners are modulated bythe terrain detail of the corresponding areas. The modulated beams aredetected by the photomultipliers 41 and 42 which convert the opticalinformation to corresponding electrical signals which afteramplification in video amplifiers 85 and 86 are supplied to the stereoviewer 82 and to the video selector 81 which perform functions alreadydescribed. The outputs of video amplifiers 85 and 86 are also appliedvia corresponding automatic gain control (AGC) stages 87 and 88 to thecorrelator circuit 90 which is a part of the height error sensing stage76, as previously described with reference to FIG. 8.

If the scanned portions of the two photographs are selectedcorresponding to the correct effective altitude, there is zero timedifference between the respective video signals from thephotomultipliers 41, 42 similar in principle to the illustration in FIG.3(a). In such a case, a zero height error is reported back to thecomputer by the error sensing circuit 76 through the A/D converter 77.However, if the scanned portions of the photographs D₁ , D₂ are notproperly positioned relative to each other, a time difference betweenthe corresponding elements of the respective video signals will bedeveloped in the manner similar to that illustrated in FIGS. 3(b) and3(c). In this case the error sensing circuit 76 through counter 77 andconverter 78 shifts the relative position of the photograph scans untilthe time difference becomes zero. This height error information indigital form from the counter 77 is utilized by the computer 50 tocorrect its previous estimate of height of the particular scannedterrain area. This information is then used in extrapolating thealtitude of the next area to be examined. For each position beingexamined, the computer utilizes suitable extrapolation from the previousaltitude values to provide the next succeeding altitude estimate. Thisinformation is further utilized by the computer 50 to prepare the codewhich controls the brightness of the beam of the altitude chart printoutcathode ray tube 64 by means of an intensity control circuit 155 whichalso controls the over-all photographic contrast of the film sheets N₁and N₂. Similarly, the orthophoto printout cathode ray tube 63 is causedto expose an image on the orthophoto film sheet N₁ corresponding to theparticular video signal from either photomultiplier 41 or 42 as selectedby the video selector 81. As a result, the terrain character is printedout on the orthophoto film sheet N₁ and the altitude data is printed outon the altitude chart film sheet N₂, hot h at the appropriately scaledterrain coordinate positions.

In another embodiment of the present invention, special "lack ofcorrelation" circuitry is incorporated in the system in order tointerrupt the automatic scanning process, when the system becomes lostdue to the inability of the analog part of the system to compensate andadjust erroneous height values approximated by the computer 50. FIG. 16shows a correlator circuit 90 coupled to receive the respective signalsdirectly from the photomultipliers 41 and 42. Assuming that theincrementally scanned areas of photographs D₁ and D₂ are improperlypositioned, the corresponding modulated signals from thephotomultipliers will be substantially different in contentcharacteristics so that when they are correlated in the correlator 90 itwill result in an output signal which is below a predetermined level setin a threshold circuit 160, resulting in a "lack of correlation" signal.Such signal is supplied to the computer 50 to interrupt the automaticcompilation process and to an alarm circuit 161 which audibly alerts theoperator. The "lack of correlation" signal is also used as the inputsignal to a blanking circuit 162 which blanks out the cathode ray tube63, thereby preventing any erroneous information from being printed outon the orthophoto film sheet N₁. The operator, upon being alerted thatthe system is lost, may examine the respective areas on the photographsD₁ and D₂ through the stereo viewer 82 and may manually guide the systemuntil it can be reset for automatic scanning.

In order that the scanning of the photographs and the printout of theorthophoto and altitude chart film sheets may proceed without the needfor stopping and starting the Y carriage of the compilation table ateach point which is to be scanned, an arrangement is provided inaccordance with one aspect of the invention which in effect arrests therelative motion of the rasters with respect to corresponding points onthe photographs and film sheets. This effect is achieved by a stopmotion circuit 165 (FIG. 17) which is arranged to provide signals to therespective deflection circuits 66-69 so as to move the correspondingelectron beams to compensate for motion of the compilation table foreach point being scanned. Use of the stop motion circuit 165 in thearrangement of the invention in FIG. 15(b) provides zero relative motionbetween the Y carriage and the rasters during examination of each areaunder the automatic mode of operation, which advantageously eliminatesthe necessity of stopping the Y carriage at each scanning cycle.

As shown in greater detail in FIG. 17, the stop motion circuit 165 maycomprise a pair of amplifiers 166 and 167 in separate channels which aresupplied a rectified rate signal via a rectifier 168 from a tachometercoupled to the Y servo 44A. The rectified signal from the Y servotachometer has an amplitude which is a linear function of the thevelocity of the Y carriage 131 (FIG. 13). Position information isobtained by integrating the rate information in the two separatechannels comprising the amplifiers 166, 167 and associated capacitors170 and 171, coupled in the feedback loops of the respective amplifiers.The outputs of the two integrating channels are coupled to thedeflection circuits of the respective cathode ray tubes, thus serving todeflect the rasters of the cathode ray tubes at an appropriate rate toachieve the desired zero relative velocity between the carriage and therasters. The output of each of the amplifiers 166 and 167 is a sawtoothwaveform representing the integral of the input of the Y carriage servostage. These respective sawtooth waveforms are interlaced as timefunctions and are respectively terminated by alternate changes in thegray code designation of table position by virtue of the control of theoscillator 173. The computer 50 controls the switch 175 to selectbetween the two waveforms; thus the rasters of the respective cathoderay tubes, including the flying spot scanners, are moved by thecorresponding deflection circuits along with the table for a particularcomputer cycle and, at the beginning of the next computer cycle, aremoved to the next desired position where the cycle repeats. This cycleis initiated in accordance with the computer timing operation which isindependent of gray code changes that appear regularly during thecontinuous motion of the compilation table. By virtue of thisadvantageous arrangement, the respective rasters are controlled tofollow the movement of the table so that the relative movement betweenthe two analog systems is arrested for particular intervals which aresynchronized with the operating cycle of the digital computer 50. Thecomputer program includes a possible delay interval to accommodate tablespeeds below the maximum permitted by the program. Thus the digital andanalog systems are kept in effective synchronism.

For proper operation of the deflection circuits in response to thetachometer signal, it is extremely desirable to be able to discharge therespective capacitors 170 and 171 rapidly at the proper time whilemaintaining a high impedance across the capacitors for the extent of thenormal operating interval in order to maintain the deflection voltageswith a high degree of stability for relatively large intervals of time,particularly during the operation of the system in the manual mode. Thecircuit of FIG. 17 includes a particular arrangement provided inaccordance with an aspect of the invention for achieving the desiredoperation. The discharging portion of the circuit comprises a pair ofenergizable discharge devices, such as the neon bulbs 176 and 177,connected across the terminals of the capacitors 170 and 171,respectively. Each neon bulb, 176 or 177, has an associated coil, 178 or179, wrapped around the bulb and coupled to an oscillator 173 in orderto establish a high frequency electromagnetic field in the vicinity ofthe selected neon bulb. The oscillator 173 is pulsed on with the adventof a change in the gray code applied from the shaft encoder 136 (FIG.15(a)) and thus provides the required electromagnetic field alternatelyat one or the other of the neon bulbs in order to ionize the bulb andprovide a low resistance discharge path across the associatedintegrating capacitor. Except when the neon bulbs 176, 177 are ionized,they provide a high impedance leakage path across the associatedcapacitor, 170 or 171, so that established charge may be maintainedthereon with reliable stability for relatively long time intervals. Withthis described arrangement of the stop motion circuit 165, the system isenabled to move the rasters of the respective cathode ray tubes inaccordance with the motion of the compilation table for each incrementalposition which is being scanned, thus enhancing the over-all accuracyand precision of the system while simplifying the position controllingmechanism of the compilation table carriages.

In still another embodiment of the present invention, the stop motion165 described above may be replaced by a stop motion circuitry 165' asshown in FIG. 18. The circuitry shown therein eliminates the use of theintegrating amplifiers 166 and 167 of FIG. 17 and the circuitsassociated therewith, and employs digital signal generating techniqueswhich exhibit greater stability when the compilation table is stationaryfor appreciable periods of time, as when the system is operated in themanual setup made. The signals necessary to cause the centers of thescanning rasters to follow the center of the incremental areas underobservation are derived from an optical coding wheel 181 which isattached to the Y lead screw 134 (FIG. 13), producing sixteen pulses foreach 0.01 inch of Y travel. The pulses energize a 5-bit counter 182which is associated with a D/A converter 183. The counter and converterare so adjusted that the most significant bit of the counter, designatedby letter e, produces a voltage required to deflect the center of therasters of the flying spot scanners bye 0.01 inch. The least significantbit, designated by letter a, produces a voltage corresponding to adeflector of 0.000625 inch. As the compilation table moves in theprofiling Y direction, the pulses from the optical coding wheel 181drive the counter, which when passing 0.01 inch switches the mostsignificant bite to a state 1, sending a signal to the computer 50.Voltages generated by the D/A converter 183 continue to increase as afunction of the count built up in the counter caused by the pulses fromthe coding wheel. However, when the computer has completed thecomputational operation of the particular cycle, a signal is sent backto the counter 182, which in effect resets the bit e to zero, thusreducing the output voltage of the D/A converter by the contribution ofthat bit, namely a voltage reduction equivalent to a 0.01 inchdeflection thereby advancing the scan to move to the next measuringposition. As shown in FIG. 18, the output voltage from the D/A converter183 is supplied to the deflection circuits 66-69 so that the scanningand printout operations of the system may be accomplished while thecompilation table moves continuously in a predetermined profilingpattern. As previously stated, the motion of the table changesdirections at the end of each Y profiling path, the counter 182 sensingsuch directional changes so that the voltage outputs of the D/Aconverter 183 have appropriate signal polarities for the particulardirections of motions of the compilation table.

The digital stop motion described above was explained with reference toa particular optical coding wheel, producing 16 pulses per 0.01 inch ofY travel, and a counter of 5 binary bits. However, it is apparent thatthe specific figures were presented for explanatory purposes only andthat other combinations may be employed to accomplish the desired stopmotion results. For example, a reversible counter operating from atwo-bit optical encoder may be used, the latter arrangement furthersimplifying the system by eliminating the need of reversing the outputpolarity of the voltages from the D/A converter 183 as the compilationtable changes direction in its profiling pattern.

It is apparent from the foregoing description that the automatic mapcompilation system in accordance with the above described embodiments ofthe present invention may be operated to develop satisfactoryphotographic maps representing image detail and height information fromrespective pairs of aerial photographs. Further, it is clear that theaccuracy of the derived information is a function of the photographsbeing scanned by a pattern that is appropriate for the particular areaunder observation at a given time. This is typically a varying pattern,namely, a skewed raster so that the instantaneous point underobservation coincides with the point printed out on the orthophoto filmsheet N₁.

As previously described, the scanning rasters of the photographs D₁ andD₂ are skewed in the raster shape modulator 75 in accordance with slopescalculated from the relative heights of previously measured areas in theneighborhood, which have been temporarily stored in the computer 50. Itis apparent therefore that the computer must be of substantial capacityand speed. Since the latter parameters are usually related to theavailability and price of the computer, and in order to increase theoverall system accuracy, raster skewing techniques which require aminimum of information from the computer were developed and incorporatedin a preferred embodiment of a raster shape modulator 75', as shown inblock diagram form in FIG. 19. For convenience, the equations (10) and(11) previously developed in connection with the raster shape modulator52 discussed above, are reproduced below:

    Δx=[G.sub.X +G.sub.2 x Z.sub.X ]ΔX+[G.sub.X +G.sub.Z xZ.sub.Y ]ΔY                                                 (10)

    Δy=[H.sub.X +H.sub.Z x Z.sub.X ]ΔX+[H.sub.Y +H.sub.z xZ.sub.Y ]ΔY                                                 (11)

In FIG. 19, the raster shape modulator 75', which is incorporated in apreferred embodiment of the present invention, comprises an X-tiltsensor 191 which is energized by the output signals from a correlatorcircuit 90", similar in performance to the correlator 90 shown in FIG.11 which serves as the height error correlator circuit by correlatingvideo signals from areas scanned as shown in FIG. 7(a). However, theoutput signals from the correlator 90" are not from a complete framebut, switched at the middle of each scanning line, and alternatelysupplied to the two differential inputs A and B of the X-tilt sensor191, as for example the correlated signals from scanned patterns asshown in FIGS. 7(c) and 7(d) are supplied to the inputs A and Brespectively of the sensor 191. The sensor 191 senses any relativeheight differentials between the incremental areas of FIG. 7(c) and 7(d)which represent the X-slope of the complete area under observation asshown in FIG. 7(a). The output signal of the sensor 191 is in distalform representing the X terrain slope component Z_(X) which may whendesired be supplied to the register 120 (FIG. 12) of the raster shapemodulator 75. Similarly, a Y-tilt sensor 192 has its two input terminalsC and D alternately connected to the output of the correlator 90".However here the switching occurs at the middle of the scanning frame sothat relative height differentials between the incremental areas ofFIGS. 7(e) and 7(f) are sensed in the Y-tilt sensor 192, resulting in adigital output signal which represents the Y terrain slope componentZ_(y) of the area under observation. The other partial derivatives ofequation (10) are generated in a generator generally designated bynumeral 193, which receives digital signals from the computer 50 andanalog signals from the compilation table 40. One analog output G_(Z) ofthe generator 193 is used as the varying reference voltage of a D/Aconverter 194 which is operated on by the digital signal Z_(X) from theX-tilt sensor 191. The analog signal from the D/A converter 194 is thenadded to an analog signal G_(X) from the generator 193 in a summationcircuit 195. The entire sum is integrated in an integrator 196, usingthe horizontal line scan timing signal from the deflection circuits inconjunction with the sync pulse generator 71 (FIG. 4) forsynchronization, and thereby obtaining a signal representing the firstexpression of the right side of equation (10).

Similarly, a signal representing the second expression of the right sideof equation (10) is gene rated by using the digital signal Z_(y) ofsensor 192 to operate on the analog signal G_(Z) in a D/A converter 197the signal output product G_(Z) Z_(Y) being added to a signal G_(Y) fromthe generator 193, in a summation circuit 198, the sum signal G_(Y)+G_(Z) Z_(Y) being integrated in an integrator 199 which is synchronizedby the vertically scan timing signal the output signal of the integratorbeing [G_(Y) +G_(Z) Z_(Y) ]ΔY. The analog output signals of theintegrators 196 and 199 are then added in a summation circuit 200 toobtain the signal Δx used as the horizontal sweep component of thescanning raster. For explanatory purposes, only the circuitry necessaryto generate the x sweep component for one photograph, namely D₁, isshown. However, it is pointed out that the raster shape modulator 52'further comprises a circuit similar to those described above, used togenerate signals representing the expressions in equation (11) therebyfurnishing the y sweep component:: for generating the complete, scanningraster of the photograph D₁. A second raster shape modulator 52'operating with somewhat modified parameters generates the scanningraster for second photograph D₂, the rasters scanning both photographsbeing skewed as a function of the various partial derivatives in theequations (10) and (11).

For a better understanding of the tilt sensors 191 and 192 reference ismade to FIG. 20 wherein both sensors are represented in a complete blockdiagram. The output of the correlator 90" is connected to one of the twoinput terminals A and B of a differential summing integrator 201 whichserves as the input subunit of the sensor 191. The output of thecorrelator 90" is sampled using a positive input for half a scan and anegative input for the remainder of the scan, switching occurring at themiddle of each scanning line (see FIGS. 7(c) and 7(d)). The switchingmay be accomplished using known signal switching techniques which arebased on electronic or electromechanical principles. If the correlator90" indicates a high error in the early part of a scan period and a lowerror in the latter half, a net voltage of corresponding polarityindicative of the direction of slope is produced at the output of theintegrator 201 and is applied to positive and negative trigger circuits202 and 203, respectively. The trigger circuits are conventionalthreshold devices, such as well known Schmitt triggers; one (202) is setso that it will trigger when the output signal of the integrator exceedsa positive threshold level set thereon while the other (203) is set soas to produce a pulse when the output of the integrator (321) is below acorrespondingly negative threshold level. When a pulse is produced byeither of the trigger circuits, a reversible counter 204 is stepped oneposition in a corresponding direction. At the same time, the particulartrigger circuit which caused the counter to advance resets the summingintegrator 201 through an OR gate 205 in order to allow a newindependent evaluation of any remaining differential heights of theincremental areas being scanned. The output of the reversible counter204 represents in digital form the previously discussed partialderivative Z_(X) which is used in the raster shape modulator 75' asshown in FIG. 19. As the reversible counter 204 advances and thescanning raster is skewed by the raster shape modulator, thedifferential output from the correlator 90" decreases. When heightdifferential in the scanned area is no longer present, the thresholdsettings in the trigger circuits 202 and 203 will not be exceeded andthe reversible counter will maintain its last value resulting in theproper X-slope value being entered into the equation (10). The operationof the Y-tilt sensor 192 shown in FIG. 20 is identical to that of theX-tilt sensor 191 described above, with the sole exception that thesignals from the correlator 90" are alternately applied to adifferential integrator 201', with switching taken place at midframe(see FIGS. 7(c) and 7(f)) so that height differentials within theincremental area being scanned in the Y direction may be detected,producing a Z_(y) digital output signal which is the Y-slope partialderivative component in equation (10).

Referring again to FIG. 19, it is apparent that the raster shapemodulator 75' diagrammed therein may generate sweep signals inaccordance with the generalized equation (10), which includes all thepartial derivative s of the photograph coordinate transformationfunction x=G(X, Y, Z). Such derivatives in addition to compensating forterrain slope as described above, may also compensate for aircraft"crabbing" along its line of flight occurring when the camera axis doesnot line up with the line of flight and for other variables such ascamera tilt and elevation differences of the airplane at the times thatthe two photographs were made.

In one specific idealized geometric interrelationship where none of theabove variations such as "crabbing" or altitude changes are present, thegeneralized equation (10) takes the following specific form;

    Δx=[f/Z-f/Z.sup.2 (X-D) Z.sub.X ]ΔX-[f/Z.sup.2 (X-D)Z.sub.y ]ΔY                                                 (12)

where, f represents the camera focal length, Z represents distance ofterrain image below the airplane; X and Y are the coordinates of thecompilation table 40, and D is the displacement of the photograph centerwith respect to machine center.

Reference is now made to FIG. 21 wherein a specific embodiment of partof the raster shape modulator 75' is shown, which is operable to producea signal represented by equation (12). In FIG. 21 the partial derivativegenerator 193 comprises a D/A converter 211 which receives from thecomputer 50 a reference voltage in the form of voltage bias which isproportional to the camera focal length f. The computer 50 also suppliesa digital signal Z which operates on the converter 211, producing ananalog output signal equal to f/Z. This voltage is in turn used as avarying reference voltage of a D/A converter 212, which is also operatedon by the digital signal Z, thereby producing an analog output equal tof/Z². The converters 211 and 212 are essentially of conventional designexcept for having digital signals operating on varying analog referencevoltages rather than on fixed reference voltages which are ordinarilypresent in conventional digital-to-analog (D/A) converters. The outputvoltage of the D/A converter 212 with its complement (as inverted in aninverter 213) are applied to an X potentiometer 215 attached to thephotograph carriage and to a manually set D potentiometer 216. Byreversing the X potentiometer input polarities, the voltage at the wiperis f/Z² multiplied by the complement of the X carriage position. D, thephoto offset distance is also multiplied by f/Z² and both signals areadded in a conventional summation circuit 218, resulting in an outputsignal represented by the quantity (f/Z²) (-X+D) which equals -(f/ Z²)(X-D). The remainder of the operation of the circuit shown in FIG. 21performs in a manner similar to the circuit shown in FIG. 17 which hasbeen described in detail. It should be noted that the circuit of FIG. 21generates only the Δx sweep components as represented by equation (12).However, the following equation

    Δy=[f/Z-(f/Z.sup.2) Y Z.sub.Y  ΔY-[(f/Z.sup.2) Y Z.sub.X ]ΔX                                                 (13)

which is a specific representation of the generalized equation (11) issimilarly generated in the raster shape modulator 75'. Thus, both the xand y sweep components are available to scan both photographs withscanning rasters which are skewed so as to account for terrain shapevariations in directions parallel and perpendicular to the line offlight.

In the foregoing description of the several embodiments of the presentinvention, it is as pointed out that correlation techniques are employedto sense errors in elevation of the incremental areas under observationas determined by the computer 50. The sensed elevation errors are thencompensated for in the analog part of the compilation system by shiftingthe relative positions of the scanning rasters of the deflectioncircuits 66 and 67 in the direction of the line of flight, which is theX coordinate of the compilation system. Correlation methods are alsoused to sense terrain slope or tilt directions and compensate for suchchanges by skewing the rasters scanning the photographs. thereby furtherincreasing the overall accuracy of the output orthophoto and drop lineinformation derived from appropriately scanning the incremental areasunder observation. However, as previously stated in some cases where theterrain tilt changes are minimal the skewing of the scanning rasters maynot be deemed necessary and therefore the raster shape modulators 75 and75' described above need not be incorporated in the system disclosedherein.

In still a further embodiment of the present invention correlationtechniques are also employed to detect and compensate for any parallaxin the aerial photographs in a direction perpendicular to the line offlight or, in other words, in the Y direction of the coordinate system.Such Y-parallax may result from relative photographic dimensionalinstability and other photographic factors which produce anunpredictable relative difference parallel to the Y-axis, of thedistances of two homologous images from the origins of their respectiverectangular coordinate systems. While normal mapping photography yieldsmodels with quite low Y-parallax, yet it is often advantageous to beable to compensate for any Y-parallax that may be present in otherwisesatisfactory stereo pairs which could not otherwise be used toaccurately extract the desired mapping information therefrom.

For a better understanding of the application of correlation techniquesto compensate for any Y-parallax present in a stereo pair, such as thephotographs D₁ and D₂ of FIG. 4, reference is made to FIGS. 22(a) and22(b), wherein there are shown line scanning patterns used in scanningincremental areas S₁ and S₂ on photographs D₁ and D₂, respectively. InFIG. 22 (a), both incremental areas S₁ and S₂ are shown as being scannedby conventional line patterns similar to the pattern shown in FIG. 7(a).The outputs of the associated photomultipliers 41 and 42 are connectedto a correlator similar to the correlator circuitry 90 of FIG. 8 usedfor height error detection. The delay lines 93 and 94 (FIG. 8) are madeprecisely equal to the fast scan period or a multiple thereof so that agiven scanning line on one photograph is compared differentially with aline above and below on the other photograph. For example, by settingthe delays 93 and 94 precisely equal to one fast scan period, thescanning line 2 of S₁ (FIG. 22(a) ) is correlated in a correlatorsimilar to correlator 92 (FIG. 8) with the scanning line 1 of S₂, whilein a correlator similar to correlator 91 line 2 of S₁ is correlated withline 3 of S₂. If there is no Y-parallax, the correlation outputs ofcorrelators 91 and 92 are substantially equal with a zero differenceoutput signal from the correlator circuit 90. If there is some parallax,one correlator (91 or 92) will show a higher output than the other withthe net difference amplified in amplifier 95, indicating a relativedrift of the two scanned areas. The output of the amplifier 95 is usedto shift the scanning rasters of the photographs D₁ and D₂ in adirection perpendicular to the line of flight, so that homologous areason the two photographs are scanned irrespective of Y-parallax present inthe photographs.

Techniques other than those described above may be used to detect anyY-parallax. For example, the areas S₁ and S₂ on the two photographs maybe scanned as shown in FIG. 22(b), wherein the vertical scan of S₂ has asmall fast component signal added to successive scans so that it takesthe form as shown. By correlating lines 1 and 1, 2 and 2, and so on ofS₁ and S₂ respectively of FIG. 22(b), and feeding the correlated outputsfrom the odd lines to one terminal of a difference amplifier, and thecorrelated outputs from the even lines to the other terminal of such adifference amplifier, the amplifier's output and polarity will bedirectly related to the magnitude and direction of the Y-parallax error.In the latter mentioned technique, the need of time delays iseliminated, since the switched scanning pattern of S₂ accomplishes thesame purpose.

In still another example of Y-parallax detection and compensationtechnique, the incremental areas S₁ and S₂ on the two photographs areconventionally scanned as shown in FIG. 22 (a). However, the scanningraster of the deflection circuit of one of the photographs is providedwith a small cyclic deflection in the Y direction at one half the framescan rate. The photomultipliers signals are passed to a singlecorrelator unit which has its output switched at the frame scan ratebetween positive and negative inputs of a differential summingintegrator. If no Y-parallax is present in the photographs, thecorrelation output signals from successive frame scans will be equal sothat the output: of the summing integrator will be substantially zero.However, if some Y-parallax is present, the correlated output signal ofone frame where the cyclic offset is in the direction to compensate theoutput signal will be greater than the output signal of the next framewhere the cyclic offset is in the opposite direction, therebyaccentuating the parallax. The result is non-cancelling input signals tothe integrator, which accumulates and amplifies any difference betweenthe two signals and applies a correction signal to the y-axis deflectioncircuit receiving the cyclic input previously mentioned.

All the foregoing embodiments of the invention have been described asincorporating scanning rasters as shown in FIG. 7(a). The rasters,defined as predetermined patterns of scanning providing substantiallyuniform coverage of a predetermined area, comprise sequentially scannedlines. The lines are scanned from left to right and top to bottom of thefrance, as represented in FIGS. 7(a) through 7(f).

As previously explained, the modulated signals generated by scanning theincremental areas of the photographs D₁ and D₂ are used in the analogpart of the system to derive height, slope, and parallax error signals,by correlating signals from predetermined portions of the frame scan.For example, a Y-axis slope error signal is derived by correlatingsignals of patterns 7(e) and 7(f). A close observation of the patternsof FIGS. 7(e) and 7(f) reveals that during the first half of the framescan only the top half of the frame is scanned while the bottom half ofthe frame is scanned during the second half of the frame scan period,making it apparent that nearly a complete frame scan period has toelapse before sufficient signals from both patterns are present so as toderive therefrom the Y-axis slope signals as explained above. Similarly,accurate height and X-axis slope error signals may only be derived aftera substantial part of the frame has been scanned. Experience hasindicated that in some systems the time necessary for scanning, beforesufficient signals are available for correlation and derivation of theerror signals, is relatively long with respect to the time available forerror analysis and computations.

Therefore, in another embodiment of the present invention a novelinterlaced scanning pattern as shown in FIGS. 23(a)-23(e) isincorporated in the automatic map compilation systems described herein.FIG. 23 (a) is a simplified diagram of a scanning pattern similar toFIG. 7(a) except for the location of the sequentially generated lineswithin the frame. Whereas in FIG. 7(a) the lines (1 through 12) scan theframe in a unidirectional, namely, from top to bottom, the lines in FIG.23(a) are located in a predetermined pattern so that alternating linesscan incremental portions of the top and bottom half frames as shown inFIGS. 23(d) and 23(c). From FIGS. 7(a) and 23(a) it is clear that eventhough the entire frame of FIG. 23(a) is covered by the same number oflines (12 lines) as the frame in FIG. 7(a), yet each group of lines inthe new scanning pattern yields a much better average of the total frameinformation than was formerly available, that error signals may bederived by employing shorter integration times, thereby shortening thetime required for error signal analysis. The predetermined pattern ofthe lines within the frame scan is controlled by the vertical sweepvoltages supplied to the deflecting circuits 66 and 67 (FIG. 4) whichcontrol the positions of the beams in the scanning rasters. In onepractical embodiment of a scanning pattern as described herein, thecomplete frame scan comprises 128 lines, with the y sweep voltage beinggenerated through a 7-bit counter 221 with an associated D/A converter222 as shown in FIG. 24(a). The output voltage of the D/A converter 222will be directly related to the binary count in the counter 221. Asshown in FIG. 24, the least bit which is energized by the sync pulsegenerator 71 (FIG. 4) for every scanning line produces a reversal in thepolarity of the output voltage of the associated D/A converter, whilethe second bit in the counter 221 causes the D/A converter 222 togenerate a voltage which will deflect the beam by one-quarter of thetotal frame height, from the frame center. The other five bits have theweights as shown in FIG. 24. A bias voltage level proportional to##EQU7## of the frame height is superposed on the output of the D/Aconverter 222 so that the first count in the counter will cause the D/Aconverter to produce a proportional voltage with a negative polarity sothat the first line scans a portion of the bottom half of the frame asshown in FIG. 23(a). The next count will produce an output voltage of1/4 plus ##EQU8## the total vertical frame sweep potential with apositive polarity causing the beam to scan a portion of the top half ofthe frame. The third count will produce the same voltage as beforeexcept for the change in polarity causing the beam to scan the bottomhalf of the frame. The entire frame will be filled after 128 steps onlythe first 12 lines being shown in FIG. 23(a), the sequentially generatedscanning lines gathering information from alternating halves of theframe. Further, the invention enables the sampling of video informationfrom a large part of the scan after only a relatively short time ascompared to a complete frame scan cycle. The above specific example ofthe scanning pattern is given for purposes of illustration only, itbeing understood that such specific values are intended to limit theinvention which enables the gathering of video information, which isequally weighted, of either side of the center frame after each pair ofline scans. The principles of the technique herein described can furtherbe used in coding transmitted classified video information so thatreceived lines may be properly oriented with respect to one another soas to produce video information of a complete frame.

Summarizing briefly, on the basis of automatic map compilation systemsthus far described, it may be stated that the computer 50 (FIG. 4)monitors, by means of position readout unit 45, the position of thetable 40 on which the pair of aerial photographs are positioned. On thebasis of the position of the table, and the calculations performedtherein in accordance with the transformation functions previouslydefined, the computer 50 calculates the respective incremental areas onthe photographs to be scanned and causes the lenses 46 and 47 to move(by energizing servos 48, 49, 52 and 53), so that the appropriateincremental areas are indeed being scanned by the rasters of the flyingspot scanners 61 and 62.

In still another embodiment of the present invention appropriatescanning of calculated incremental areas is achieved by the computer 50controlling, rather than monitoring the position of table 40, so thatlenses 46 and 47 may be in fixed positions similar to lenses 83 and 84thereby eliminating the need of servos 48, 49, 52 and 53, furtherreducing the storage and computational capacity of the computer 50.

Reference is now made to FIG. 25, which is a simplified block diagram ofan arrangement wherein the computer 50 controls the exact position ofthe table 40 for proper compilation of mapping information. Aspreviously stated, the computer 50 calculates the desired position ofthe table and supplies the signals through a D/A converter 331 to adifferencing amplifier 332 which is also energized through a D/Aconverter 333 by signals from an optical encoder 334 that continuouslymonitors the exact position of the table. The difference between theanalog outputs of the converters 331 and 333 is the instantaneous errorin the position of the table. This error is then supplied by amplifier332 to the servo, controlling the movement of the table so that thepositioning error is eliminated. The output of amplifier 332 is alsosupplied to the deflection circuits 66 and 67 so that the rasterstherein may be appropriately shifted to compensate for any error stillpresent in the position of the compilation table.

In still another embodiment of the present invention, the computer 50controls the position of the compilation table 40 by an arrangementsimilar to the one shown in FIG. 25 except that the optical encoder 334energizes the D/A converter 333 through a reversible counter (not shown)which is periodically "read" and later reset by the computer. On thebasis of the reading, the computer determines the magnitude of error inthe position of a first point, and calculates the magnitude of thedigital signal necessary to supply to D/A converter 331 in order to movethe table to the second point selected for scanning. In either of thelast two embodiments of the present invention wherein the computercontrols the table position, the optical encoder 334 and the D/Aconverters 331 and 333 are of sufficient sensitivity and capacity so asto produce error signals corresponding to the desired positioningaccuracy of the system. Further, it should be apparent that in either ofthe last two embodiments of the present invention wherein the computercontrols the table position and wherein the lenses 46 and 47 are infixed positions, only photographs taken under optimum conditions, namelyvertical photography and both taken from the same altitude, may be used,since the only variable means for the computer 50 to direct scanners 61and 62 to the computed homologous areas is by deflecting the beams ofthe scanners, such deflection being of relatively limited range.

In still another embodiment of the present invention, each of thestereoscopic photographs D₁ and D_(z) and the film sheets N₁ and N₂shown in FIG. 4 as mounted on the table 40, are actually mounted onindividual compilation tables 341-344 respectively as shown in FIG. 26.The operator console 340 shown therein, includes the various circuitsnecessary for the manual and mark modes of the system as previouslyexplained, and the computer 50 separately controls each of the fourtables.

Briefly, tables 343 and 344 are moved in the fixed profiling pattern, sothat the orthophoto and the altitude chart may be printed thereon.However, during each cycle of operation of the computer, the computer,instead of moving the scanning optics over the two photographs as aresult of the estimated height and locations of the homologousincremental areas, moves the tables 341 and 342. Then, the scanning isperformed and error signals produced, to indicate to the computer theerror in the estimates, computed for the particular scanned incrementalareas as representing homologous imagery. This signal is used by thecomputer to update the information therein for subsequent estimates ofthe locations of another pair of incremental areas as representinghomologous imagery. Thus, it can be generally stated that in theembodiments in which the two stereoscopic photographs are supported on asingle table, such as that designated by numeral 40 in FIG. 4, thecomputer, as a result of the estimates computed therein, moves thescanning optics with respect to the two photographs so that portionsthereof, estimated to represented homologous imagery, are scanned.However, in the arrangement as shown in FIG. 26 in which the twophotographs are supported on separate tables (341 and 342), the computermoves these tables with respect to the scanning system so that a fixedscanning system is located above the portions of the photographsassumed, or estimated, by the computer as representing homologousimagery. Also a system greatly increases the overall versatility of thesystem in that photographs with different dimensions and printout filmsheets of preselected dimensions may be utilized. For example, thepreparation of aerial mosaic s defined as the compilation of individualaerial photographs fitted together systematically to form a compositeview of an entire area covered by the photographs may be greatlysimplified, by using large photographs, each pair printing out only arelatively small area on the orthophoto and altitude photo sheets sothat a single large mosaic is automatically produced.

From the foregoing detailed description, it should be apparent thatapplicant has herein disclosed a novel automatic compilation employing adigital computer and an analog subsystem. While the invention has beendescribed in conjunction with automatic map compilation developing anorthophotograph and an altitude chart from information derived from apair of stereoscopic photographs, it is clear that the system is notlimited thereto; rather, the principles described above may be utilizedin many other ways.

Furthermore, by virtue of the inclusion of a computer in the system,added versatility is provided since modifications in the computerprogram permit the adaptation of the system to the solution of problemsrelated to the information in the stereoscopic photographs. Problemsrelated to "cut and fill" in construction projects over large areas, orthe selection of road paths between remote points, may be convenientlysolved by the automatic system described herein. For example, the heightinterval information provided in the presently described system in theforth of an altitude chart may be simultaneously stored on any knowncomputer storage media such as magnetic tape, and later used todetermine the height of all incremental areas within a predeterminedarea above and below a selected reference, thereby automaticallycalculating the amount of cut and fill required to level the area at aheight equal to the selected reference. As to the selection of preferredroadpaths between predetermined points, the computer may beappropriately programmed by properly selecting coefficients of thetransformation functions previously described so that the apparent lineof flight of the airplane taking the stereoscopic photographs isappropriate to make the predetermined points appear to be at the samealtitude, thereby introducing apparent differential height values in allintermediate points so that a preferred route between the points may beautomatically determined.

The height interval information provided in this system in the form ofthe altitude chart may also be used in other associated equipment bydirectly applying the derived information to control the movement of amilling machine or other cutting tool, for example, to make reliefmodels or reproduce a particular contour or desired pattern.

Although there has been described above a specific arrangement of anautomatic map compilation system in accordance with the invention forthe purpose of illustrating the manner in which the invention may beused to advantage, it will be appreciated that the invention is notlimited thereto. Accordingly, any and all modifications, variations orequivalent arrangements falling within the scope of the annexed claimsshould be considered to be a part of the invention.

What is claimed is:
 1. An automatic map compilation system, for derivinginformation from a pair of stereoscopic photographs by scanning portionsthereof which represent homologous imagery comprising:first means forscanning at least portions of a pair of stereoscopic photographs anddeveloping video signals in accordance therewith; second means includingcomputing means, operable in a sequence of cycles of operation, forestimating for successive cycles of operation, on the basis of at leastphotogrammetric data related to said pair of photographs, the portionsof said photographs which represent homologous imagery and controllingsaid first means to synchronously scan said portions; and third meansresponsive to said video signals for comparing said video signalstherein and producing error signals which represent the degree of errorin said second means in estimating said portions of said photographs asrepresenting homologous imagery said second means including meansresponsive to said error signals for updating the estimate of thelocations of portions of said photographs, which represent homologousimagery, adjacent the scanned portions.
 2. An automatic map compilationsystem for deriving information from a pair of stereoscopic photographsby scanning portions thereof which represent homologous imagerycomprising:first means for scanning at least portions of a pair ofstereoscopic photographs and developing video signals in accordancetherewith; second means, including computing means for storingphotogrammetric data related to said pair of stereoscopic photographsand means for cyclically estimating for each cycle of operation of saidsecond means, the locations of portions of said photographs whichrepresent homologous imagery and for moving said photographs withrespect to said first means, so that said first means synchronously scanthe portions, the locations of which were estimated by said computingmeans; and third means responsive to said video signals for correlatingthe signals therein and producing error signals which indicate an errorin the estimates computed in said second means, said second means beingresponsive to said error signals to update the estimates computedtherein.
 3. An automatic map compilation system, for derivinginformation from a pair of stereoscopic photographs by scanning portionsthereof selected as representing substantially homologous imageryfirstmeans for scanning a pair of stereoscopic photographs and developingvideo signals in accordance therewith; computing means, including meansfor storing photogrammetric data related to said pair of stereoscopicphotographs and means for estimating, on the basis of saidphotogrammetric data and information derived during scanning contiguousportions of said photographs, for each cycle of operation of saidcomputing means, portions of said pair of stereoscopic photographssubstantially representing homologous imagery, said computing meansfurther including means for controlling the positions of saidphotographs with respect to said first means, so that said first meansmeans said estimated portions; and second means responsive to said videosignals for comprising said video signals therein, and producinginformation signals which are related to the degree of error in theselection of said portions as representing homologous imagery.
 4. Anautomatic map compilation system for deriving data from a pair ofstereoscopic photographs by scanning incremental areas thereofcomprising:means for scanning a pair of stereoscopic photographs of anobject and developing video signals in accordance therewith; computingmeans for controlling the positions of said means for scanning withrespect to said pair of stereoscopic photographs to cause synchronousscanning of a first pair of incremental areas on said pair ofstereoscopic photographs, said incremental areas having been selected onthe basis of photogrammetric data including estimated height related tosaid pair of stereoscopic photographs as representing homologous objectimagery; means responsive to said video signals for analyzing said videosignals therein and producing error signals which represent the degreeof error in selecting said first pair of incremental areas asrepresenting homologous object imagery; and means for energizing saidcomputing means with said error signals for updating therein saidestimated height of the homologous object imagery represented in saidfirst pair of incremental areas and for use in selecting a second pairof incremental areas on said pair of stereoscopic photographs asrepresenting homologous object imagery.
 5. An automatic map compilationsystem for deriving terrain height information from a pair ofstereoscopic photographs of terrain, by scanning incremental areasthereof, comprising:means for scanning a pair of stereoscopicphotographs of terrain and developing video signals in accordancetherewith; computing means for controlling said means for scanning tocause synchronous scanning of a first pair of incremental areas on saidpair of stereoscopic photographs, the locations of said incrementalareas one said pair of stereoscopic photo having been computed on thebasis of photogrammetric data including estimated height related to saidpair of stereoscopic photographs as representing homologous terrainimagery; means responsive to said video signals for analyzing said videosignals therein and producing error signals which represent the degreeof error in computing said first pair of incremental areas asrepresenting homologous terrain imagery; and means for energizing saidcomputing means with said error signals for updating therein saidestimated height of the homologous terrain imagery represented in saidfirst pair of incremental areas and for use in computing the locationsof a second pair of incremental areas on said pair of stereoscopicphotographs as representing homologous terrain imagery.
 6. An automaticmap compilation system for recording terrain data, derived from scanningincremental areas of a pair of stereoscopic photographs of terraincomprising:means for scanning a pair of stereoscopic photographs ofterrain and developing video signals in accordance therewith; computingmeans for controlling said means for scanning to cause synchronousscanning of substantially homologous incremental areas on saidphotographs in accordance with preprogrammed photogrammetric datarelated to said pair of stereoscopic photographs; means responsive tosaid video signals for developing scanning error signals for adjustingthe relative incremental areas Of the photographs being synchronouslyscanned, said scanning error signals also energizing said computingmeans for further controlling said means for scanning said pair ofphotographs; and output means for recording terrain data derived fromthe synchronously scanned homologous areas of said pair of stereoscopic:photographs.
 7. An automatic map compilation system for deriving datafrom a pair of stereoscopic photographs by scanning incremental areasthereof, which represent homologous terrain imagery comprising:means forscanning a pair of stereoscopic photographs of terrain and developingvideo signals in accordance therewith; computing means for controllingsaid means for scanning to cause synchronous Scanning of a first pair ofincremental areas on said pair of stereoscopic photographs, saidincremental areas having been computed by said computing means on thebasis of photogrammetric data including estimated height related to saidpair of stereoscopic photographs as representing homologous terrainimagery; means responsive to said video signals for analyzing said videosignals therein and producing error signals which represent the degreeof error in said computing means in computing said first pair ofincremental areas as representing homologous terrain imagery; means forenergizing said computing means with said error signals for updatingtherein estimated height of the homologous terrain imagery representedin said first pair of incremental areas and for computing a second pairof incremental areas on said pair of stereoscopic photographs asrepresenting homologous terrain imagery; and output means for recordingdata related to the terrain represented by said homologous terrainimagery from signals derived from the scanning of said first pair ofincremental areas.
 8. An automatic map compilation system for derivingdata from a pair of stereoscopic photographs of terrain by scanningincremental areas thereof, comprising:means for scanning a pair ofstereoscopic photographs of terrain and developing video signals inaccordance therewith; computing means for controlling said means forscanning to cause synchronous scanning of a first pair of incrementalareas on said pair of stereoscopic photographs, said incremental areashaving been computed by the computing means on the basis ofphotogrammetric data related to said pair of stereoscopic photographs asrepresenting homologous terrain imagery; means responsive to said videosignals for analyzing said video signals therein and producing errorsignals which represent the degree of error in said computing means incomputing said first pair of incremental area as representing homologousterrain imagery; means for energizing said computing means with saiderror signals for updating therein estimated height of the homologousterrain imagery represented in said first pair of incremental areas andfor computing a second pair of incremental areas on said pair ofstereoscopic photographs as representing homologous terrain imagery; andmeans for recording data of terrain represented by said homologousterrain imagery in response to said video signals developed duringscanning of said first pair of incremental areas, and in response tosignals representing updated estimated height from said computing means.9. In an automatic map compilation system in which, incremental areas ofa pair of stereoscopic photographs, supplied to said system, arm scannedto derive information therefrom, the improvement comprising:means forscanning a pair of stereoscopic photographs; computing means forcontrolling positions of said means for scanning with respect to saidphotographs to cause synchronous scanning of portions thereof inaccordance with photogrammetric information stored by said computingmeans; means for generating signals corresponding to imagery in thescanned portions of the photographs; and means responsive to said videosignals for developing scanning error signals for adjusting the relativeincremental areas of the photographs being synchronously scanned, saidscanning error signals also energizing said computing means for furthercontrolling the homologous incremental areas on said photographs forsynchronous scanning.
 10. An automatic map compilation system forprinting out information derived from scanning portions of a pair ofstereoscopic photographs of terrain comprising:means for scanning a pairof stereoscopic photographs; computing means for controlling positionsof said means for scanning with respect to said photographs to causesynchronous scanning of portions thereof in accordance withphotogrammetric information stored by said computing means; means forgenerating video signals corresponding to imagery in the scannedportions of the photographs; means responsive to said video signals fordeveloping scanning error signals for adjusting the relative incrementalareas of the photographs being synchronously scanned, said scanningerror signals also energizing said computing means for furthercontrolling the subsequent selection of portions of said photographs forsynchronous scanning; and means for printing out an orthophotocorresponding to said generated signals and an altitude chartcorresponding to the height of the terrain represented in thestereoscopic photographs substantially concurrently with the operationof scanning.
 11. An automatic map compilation system for printing outterrain data by scanning portions of a pair of stereoscopic photographsof terrain comprising:scanning means for scanning a pair of stereoscopicphotographs; computing means including means for storing photogrammetricinformation related to said pair of stereoscopic photographs forcontrolling positions of said means for scanning with respect to saidphotographs to cause synchronous scanning of portions thereof inaccordance with the photogrammetric information stored by in saidcomputing means; means for generating signals corresponding to imageryin the scanned portions of the photographs; means including correlatingcircuitry responsive to said generated signals for developing errorsignals, said scanning means being responsive to said error signals fordeflecting said scanning means so as to change the positions of thecenters of the portions of said photographs which are synchronouslyscanned so that substantially homologous incremental areas of thephotographs are synchronously being scanned; and means for printing outan orthophoto corresponding to said generated signals and an altitudechart corresponding to the height of the terrain represented in thestereoscopic photographs, substantially concurrently with the operationof scanning.
 12. An automatic map compilation system for printing out anorthophoto and an altitude chart from signals derived by scanningincremental areas of a pair of stereoscopic photographs comprising:meansfor scanning a pair of stereoscopic photographs and developing videosignals in accordance therewith; computing means for controlling saidmeans for scanning to cause synchronous scanning of substantiallyhomologous incremental areas on said photographs in accordance withphotogrammetric data stored therein related to the pair of stereoscopicphotographs; means responsive to said video signals for developingscanning error signals for adjusting the relative incremental areas ofthe photographs being synchronously scanned, said scanning error signalsalso energizing said computing means for further controlling thehomologous incremental areas on said photographs for synchronousscanning; and means for printing out an orthophoto corresponding to saidgenerated signals and an altitude chart corresponding to the height ofthe terrain represented in the stereoscopic photographs substantiallyconcurrently with the operation of scanning.
 13. An automatic mapcompilation system for recording information derived by scanningincremental areas of a pair of stereoscopic photographs comprising:meansfor scanning a pair of stereoscopic photographs and developing videosignals in accordance therewith; computing means for controlling saidmeans for scanning to cause synchronous scanning of substantiallyhomologous incremental areas on said photographs in accordance withphotogrammetric data stored therein related to the pair of stereoscopicphotographs; means responsive to said video signals for developingscanning error signals for adjusting the relative incremental areas ofthe photographs being synchronously scanned, said scanning error signalsalso energizing said computing means for further controlling thehomologous incremental areas on said photographs for synchronousscanning; and means for incrementally recording image detail from saidsynchronously scanned homologous incremental areas and altitudeinformation derived from the synchronously scanned homologous areas ofthe stereoscopic photographs.
 14. An automatic map compilation systemfor deriving printable information by scanning incremental areas of apair of stereoscopic photographs comprising:scanning means forelectronically scanning a pair of stereoscopic photographs; means forgenerating electrical signals corresponding to imagery in scannedportions of said photographs; means for correlating said electricalsignals to detect relative time displacements therebetween and producingheight error signals in accordance therewith; means responsive to saidheight error signals for modifying the portions of said photographsbeing synchronously scanned to establish time coincidence between saidelectrical signals generated during the synchronous scanning of theportions of said photographs; a computer responsive to said height errorsignals for calculating altitude information of the portions of saidphotographs being synchronously scanned in accordance therewith and withphotogrammetric information stored therein, said computer furthercontrolling relative portions of said photographs to be scanned; firstprintout means coupled to receive said electrical signals for providinga record in orthographic projection corresponding to said stereoscopicphotographs; and second printout means coupled to the computer forproviding: a record indicative of the altitudes of the record printedout by said first printout means.
 15. An automatic map compilationsystem for deriving printable information by scanning portions of a pairof stereoscopic photographs comprising:positioning means for positioninga pair of stereoscopic photographs and a photosensitive film sheet inpredetermined fixed relationships to each other; scanning means forelectronically scanning predetermined portions of said stereoscopicphotographs, said portions defining centers; means for moving saidpositioning means in a predetermined pattern; means for generatingelectrical signals corresponding to imagery in scanned portions of saidphotographs; means for correlating corresponding portions of saidgenerated electrical signals to detect relative time displacementstherebetween, and producing height error signals in accordancetherewith; means responsive to said height error signals forindependently varying the positions of the centers of the scannedportions of said photographs relative to each other, to establish timecoincidence of said generated electrical signals; a computer forcalculating the altitudes of the imagery in the scanned portions of saidphotographs in accordance with photogrammetric data stored therein, andsaid height error signals supplied thereto; and printout meansresponsive to the altitudes calculated in said computer for printing outsaid altitudes on said photosensitive film sheet.
 16. An automatic mapcompilation system for printing out information derived from scanningportions of a pair of stereoscopic photographs comprising:support meansfor supporting a pair of stereoscopic photographs and a photosensitivefilm sheet in predetermined fixed positions thereon; scanning means forelectronically scanning predetermined portions of said stereoscopicphotographs; means for moving said support means in a predetermine&fixed pattern; generating means for generating electrical signalscorresponding to imagery in scanned portions of said photographs; meansfor correlating corresponding portions of said generated electricalsignals to detect relative time displacements therebetween and toproduce height error signals in accordance there-. with; a computerresponsive to said height error signals and to photogrammetricinformation stored therein for controlling the synchronous scanning ofsubstantially homologous portions of said stereoscopic photographs; andprintout means responsive to said generated electrical signalscorresponding to the scanned portions of said photographs printing outon said photosensitive film sheet image elements, so as to provide arecord in orthographic projection corresponding to said stereoscopicphotographs.
 17. An automatic map compilation system for printing outinformation on photosensitive film sheets, said information outinformation on photosensitive film sheets, said information beingderived by scanning incremental areas of a pair of stereoscopicphotographs comprising:a movable table on which a pair of stereoscopicphotographs and a pair of photosensitive film sheets to be exposed inaccordance with information derived from photographic detail in thestereoscopic photographs are adapted to be placed in fixed relationshipto each other; means for automatically moving said movable table in apredetermined pattern; scanning means for scanning each of saidphotographs with an individually controllable raster; movable lens meansfor directing the scanning rasters of said scanning means to selectedincremental areas of said photographs; a computer responsive to signalscorresponding to the position of said movable table which is related tosaid pair of stereoscopic photographs and to photogrammetric data storedtherein for directing the scanning rasters, which scan said photographs,to selected areas thereof; means for modifying the photogrammetric datain accordance with the actual positions of the photographs as determinedduring an alignment process preparatory to automatically moving saidmovable table in said predetermined pattern; means for generatingsignals corresponding to imagery in the scanned areas on thephotographs; means responsive to said generated signals for correlatingcorresponding portions of the signals to detect relative timedisplacements therebetween; error detecting means responsive to thesignals having relative time displacements for developing height andtilt error signals and applying them to said scanning means so as tovary the relative positions and shapes of the scanning rasters on thestereoscopic photographs in order to substantially eliminate thedetected relative tinge displacements between the portions of saidsignals and establish time coincidence therebetween, said computerfurther being energized by said height error signals to correct thepreviously approximated altitude of the incremental area represented bythe said incremental areas being scanned and further control theincremental scanned areas on the next scanning cycle by incorporatingthe corrected altitude in computations performed therein; first printoutmeans responsive to the corrected calculated altitudes in said computerfor incrementally printing out on one of said photosensitive film sheetsinformation corresponding to altitudes of said scanned incrementalareas; and second printout means responsive to said generated signalscorresponding to the scanned areas on the photographs for incrementallyprinting out on the other of said photosensitive film sheets imageelements so as to provide a record in orthographic projectioncorresponding to said stereoscopic photographs.
 18. In an automatic mapcompilation system, of the type in which a pair of stereoscopicphotographs, supplied thereto, are positioned for scanning to derivedata by scanning portions thereof, an alignment arrangementcomprising:means for positioning a pair of stereoscopic photographs withrespect to one another; means for electronically scanning said pair ofstereoscopic photographs; means for generating video signalscorresponding to imagery in the scanned portions of the photographs; acomputer connected to control the scanning means; means for applyingappropriate photogrammetric information related to the pair ofstereoscopic photographs to the computer for storage therein; means formanually controlling the computer to cause the scanning of particularselected portions of the photographs; remote electronic viewing meanscoupled to display a representation of the selected portions to anoperator for use in aligning the corresponding selected portions of therespective photographs in order to compensate for any misalignment inthe acutal positioning of said photographs with respect to one another;means for supplying signals indicative of the coordinates of theselected portions of the photographs to the computer in order to modifythe stored photogrammetric information stored therein; and means forprinting selected marks on associated photosensitive sheets to designatepositions corresponding to selected portions of the photographs.
 19. Anautomatic map compilation system for deriving information from a pair ofstereoscopic photographs supplied thereto by scanning incremental areasthereof comprising:movable means for transporting and moving in apredetermined profiling pattern a pair of stereoscopic photographs fixedin position with respect to one another; position indicating means fordeveloping signals indicative of the position of said movable means withrespect to predetermined references; electronic scanning meansindividually associated with each of said stereoscopic photographs forscanning incremental areas of the photographs with controllable rasters;a computer for controlling synchronous scanning of substantiallyhomologous incremental areas of the photographs in accordance withspecific photogrammetric data related to the particular stereoscopicphotographs and their relative positions on said movable means; meansresponsive to signals from the computer for directing the respectivescanning rasters to the selected incremental areas on the photographsfor synchronous scanning; means for generating video signalscorresponding to imagery in the scanned areas of the photographs; errorsensing means including correlating means responsive to said generatedsignals to detect relative time displacements therebetween, fordeveloping height error signals in response thereto, said height errorsignals being supplied to said scanning means to cause a relativedisplacement between said scanning rasters, so as to minimize therelative time displacements between said correlated video signals, saidcomputer further being energized by said height error signals to; adjustthe previously estimated altitude of the incremental areas being scannedand further incorporate said adjusted altitude in calculating theposition of homologous incremental areas on the photographs to besynchronously scanned; and means for printing out photographic detailderived from said video signals in orthographic relationship andaltitude information pertaining to the scanned photographs, derived fromthe computer.
 20. The system defined by claim 19 furthercomprising:means responsive to the motion of said movable table forgenerating a signal indicative of the motion thereof; and meansresponsive to said motion indicating signal for generating deflectingsignals so as to cause substantially zero displacement between thecenters of the scanning rasters generated by said electronic scanningmeans and the centers of the incrementally scanned areas, while, saidphotographs are moved in said predetermined profiling pattern,
 21. Thesystem defined by claim 19 further comprisingraster shaping meansenergized by signals representing height and tilt information of saidincremental areas being scanned ant further energized by saidpositioning indicating means to produce compensating signalscorresponding to tilt components of said scanned incremental areas andto energize said scanning means therewith for controlling the sizes andshapes of said scanning rasters so as to scan substantially homologousincremental areas.
 22. The system defined by claim 19 furthercomprising:means for applying a servo error signal from said meansresponsive to signals from the computer to said electronic scanningmeans to deflect said scanning rasters to compensate for errors in thepositioning of said means responsive to signals from the computer. 23.The system defined by claim 19 wherein said error sensing means furtherincludes means responsive to said generated video signals to eliminatepredetermined frequency components therefrom, and to detect relativetime displacements therebetween, for developing height error signals inresponse thereto in combination with the signals developed by saidcorrelating means included in said error sensing means, said heighterror signals being supplied to said scanning means to cause a relativedisplacement between said scanning rasters so as to minimize therelative time displacements between said video signals.
 24. In anautomatic map compilation system of the type, scanning portions of apair of stereoscopic photographs supplied thereof, to derive informationtherefrom, an arrangement, comprising:first and second movable means fortransporting and moving a pair of stereoscopic photographs; electronicscanning means associated with each of said stereoscopic photographs forscanning at least portions thereof and developing video signals inaccordance with the imagery contained therein; computing means forcyclically estimating for each cycle of operation of said computer meansestimating on the basis of photogrammetric data related to said pair ofstereoscopic photographs stored in said computing means positions ofportions of said photographs which represent homologous imagery andcontrolling said first and second movable means to move said photographswith respect to said electronic scanning means to cause the synchronousscanning of said portions which represent homologous imagery; errorsensing means responsive to said video signals for analyzing said videosignals therein and producing error signals which represent the degreeof error of said computing means in estimating said portions asrepresenting homologous imagery; and means for energizing said computingmeans with said error signals so as to update the photogrammetric datarelated to said portions which have been synchronously scanned, andstore said updated data therein.
 25. The system defined by claim 24further including:third movable means responsive to controlling signalsfrom said computing means for transporting and moving a material havinga photosensitive surface; and output means responsive to said videosignals for producing an orthographic recording of imagery in said pairof stereoscopic photographs by exposing a portion of said materialhaving a photosensitive surface with light signals corresponding to saidvideo signals which are developed by said electronic scanning means. 26.The system defined by claim 24 further including:means responsive tocontrolling signals from said computing means for transporting andmoving a material having a photosensitive surface; and output meansresponsive to signals from said computing means, said signalsrepresenting updated altitude information derived from said updatedphotogrammetric data related to said portions of said pair ofstereoscopic photographs which have been synchronously scanned, forproducing an altitude chart represented by the imagery in said pair ofstereoscopic photographs by exposing a portion of said material having,a photosensitive surface with said signals.
 27. In an automatic mapcompilation system, in which, incremental areas, of a pair ofstereoscopic photographs supplied thereto, are scanned to deriveinformation therefrom the arrangement comprising:table means formounting and moving at least a pair of stereoscopic photographs in apredetermined profiling pattern said photographs being fixedly mountedon said table means; means including computing means and scanning meansfor scanning portions of said at least pair of stereoscopic photographswith rasters of predetermined patterns and configurations; and meansresponsive to signals derived from said table means for energizing saidscanning means so that the centers of said rasters substantially followthe centers of said portions of said photographs as controlled by saidcomputing means, as said photographs are being moved by said table meansin said predetermined profiling pattern.
 28. In a mapping system anarrangement for synchronizing the position of an electronicallydeflected scanning beam to follow a moving object to be scanned duringparticular repetitive intervals comprising:means for generating anelectrical signal indicative of the motion of said object; dual channelintegrators for integrating said signal; means for alternativelydischarging said integrators as said moving object passes predeterminedpoints; and means for alternatively energizing said electronicallydeflected beam with the outputs of the respective integrators insynchronism with independent timing pulses by switching between theoutputs thereof so as to deflect the beam in accordance with theselected integrator output.
 29. In a mapping system of the type whereininformation is derived from scanning an object, an arrangement forcontrolling scanning means which scan with a raster of a predeterminedconfiguration a portion of a movable object so that the center of saidraster follows the center of the portion of said movable objectincluding:Optical encoding means for detecting movement of said movableobject and producing signals in accordance therewith; and meansresponsive to said signals for producing output signals whosecharacteristics are related to the movement of said movable object andto energize said scanning means therewith so that the center of theraster of said scanning means substantially follows the center of thescanned portion of said object.
 30. In an automatic map compilationsystem, wherein video signals are produced by electronically scanningincremental areas of a pair of stereoscopic photographs supplied theretosaid incremental areas being selected by a computer for each cycle ofoperation thereof on the basis of photogrammetric data supplied theretoand altitude data previously updated therein so as to scan substantiallyhomologous incremental areas, the arrangement comprising:error sensingmeans including first and second correlating means, said firstcorrelating means being responsive to said generated video signals todetect relative time displacements therebetween and producing a firstheight error sensing signal in response thereto, said second correlatingmeans being responsive to a selected frequency bandwidth of thegenerated video signals for detecting relative time displacementstherebetween and developing a second height error signal, said errorsensing means further including means for combining said first andsecond height error signals and producing an output height error signalwhich is indicative of the degree of error in the selection of saidincremental areas as representing homologous areas by said computer. 31.In an automatic map compilation system, wherein video signals areproduced by scanning with scanning means in predetermined rasterpatterns and configurations incremental areas of a pair of stereoscopicphotographs supplied thereto, said incremental areas having beenselected on the basis of photogrammetric data related to said pair ofphotographs and data related to contiguous incremental areas bycomputing means as representing homologous imagery, the arrangementcomprising:height error sensing means responsive to said video signalsfor correlating the signals therein, and developing height error signalswhich are indicative of the degree of error in the selection of saidincremental areas by said computing means as representing homologousimagery; and electronic raster shape means responsive to said videosignals and including means for switching said signals therein in apredetermined timing sequence as a function of said predeterminedconfigurations of said rasters for producing slope-defining signals as afunction of detected incremental altitude variations within saidincremental areas being scanned, said computing means being adapted tobe energized by said signals indicative of incremental altitudevariations within said incremental areas being scanned.
 32. In anautomatic map compilation system, wherein video signals are produced byscanning with scanning means in predetermined raster patterns andconfigurations, incremental areas of a pair of stereoscopic photographs,supplied to said, the incremental areas being selected as representinghomologous imagery by computing means on the basis of photogrammetricdata related to said pair of photographs, and data related to contiguousincremental areas, the arrangement comprising:height error sensing meansresponsive to said video signals for correlating the signals therein anddeveloping height error signals which are related to the degree of errorin the selection of said incremental areas by said computing means asrepresenting homologous imagery; electronic raster shape meansresponsive to said video signals and including means for switching saidvideo signals therein in a predetermined timing sequence as a functionof said predetermined configurations of said rasters for producing slopesignals as a function indicative of incremental altitude variationswithin said incremental areas being scanned; and means responsive tosaid slope for energizing said scanning means so that the configurationsof said rasters may be shaped in accordance therewith, thereby furthercontrolling the raster configurations for scanning said incrementalareas of said pair of stereoscopic photographs which representhomologous imagery.
 33. In an automatic mapping system, in which a pairof stereoscopic terrain photographs supplied thereto are scanned torecord information therefrom, the combination comprising:means forpositioning a pair of stereoscopic terrain photographs in a controllablerelationship to one another including means for moving said pair ofstereoscopic terrain photographs along a predetermined profiling patternwith respect to preselected references; means for electronicallyscanning said photographs; means for generating video signalscorresponding to terrain imagery in portions of the scanned photographs;a digital computer coupled to control the scanning means; correlationcircuitry responsive to said video signals for developing height errorsignals; raster shaping means energized by signals from said computercorresponding to height and tilt information stored therein and furtherenergized by signals from said positioning means corresponding to theposition of said pair of photographs with respect to said preselectedreferences for developing tilt compensating signals for energizing saidscanning means to control the size and shape of the scanned portions ofsaid photographs so that they correspond to substantially homologousterrain incremental areas; and means for recording altitude and terrainphotographic information derived from the synchronously scannedhomologous areas of the stereoscopic photographs.
 34. In an automaticmapping system, in which a pair of stereoscopic terrain photographssupplied thereto are scanned to record information therefrom, thecombination comprising:means for positioning a pair of stereoscopicphotographs in a controllable relationship to one another includingmeans for moving said pair of stereoscopic photographs along apredetermined profiling pattern with respect to preselected references;means for electronically scanning said photographs; . means forgenerating video signals corresponding to imagery in portions of thescanned photographs; a digital computer coupled to control the scanningmeans; correlation circuitry responsive to said video signals fordeveloping height error signals; and raster shaping means energized bysignals from said computer corresponding to height and tilt informationstored therein and further energized by signals from said positioningmeans corresponding to the position of said pair of photographs withrespect to said preselected references for developing tilt compensatingsignals for energizing said scanning means to control the size and shapeof the scanned portions of said photographs so that they correspond tosubstantially homologous incremental areas.
 35. In an automatic mapcompilation system, wherein video signals are produced by electronicallyscanning incremental areas of a pair of stereoscopic photographssupplied thereto, said incremental areas being selected by a included insaid system on the basis of prestored photogrammetric data and altitudedata previously updated, so as to scan substantially homologous areas,said video signals being correlated to produce height and tilt errorsignals, said height error signals being used to vary the relativepositions of the scanned areas so as to synchronously scan substantiallyhomologous areas, the arrangement comprising:raster shaping meansenergized by said tilt error signals corresponding to tilt components ofthe incremental areas being scanned, and further energized by positionindicating means which relate the positions of the photographs withrespect to predetermined references to said prestored photogrammetricdata for producing tilt compensating signals to control the sizes andshapes of the incremental areas being scanned as a function of effectivetilt directions of the incremental areas being scanned.
 36. In anautomatic map compilation system wherein a pair of stereoscopicphotographs are scanned to derive information therefrom the combinationcomprising:means for electronically scanning a pair of stereoscopicphotographs; means for generating video signals corresponding to imageryin scanned portions of the photographs; a digital computer forcontrolling, for each cycle of operation thereof the portions of thephotographs to be scanned in accordance with preprogrammedphotogrammetric data related to said pair of stereoscopic photographs,stored therein; and correlation circuitry responsive to said videosignals for developing error signals, said circuitry including thresholdcircuit means responsive to a preselected correlation level forproviding an inhibiting signal when the correlation of said videosignals falls below said selected level.
 37. An automatic mapcompilation system adapted to derive information from a pair ofstereoscopic photographs supplied thereto comprising:movable means fortransporting and moving a pair of stereoscopic photographs; positionindicating means for developing signals indicative of the position ofsaid movable means with respect to predetermined references; electronicscanning means individually associated with each of said photographs forscanning incremental areas of the photographs with controllable rasters;a computer for controlling synchronous scanning of substantiallyhomologous incremental areas of the photographs in accordance withspecific photogrammetric data related to the particular stereoscopicphotographs and their positions on said movable means; means responsiveto signals from the computer for directing the respective scanningrasters to the selected incremental areas on the photographs forsynchronous scanning; means for generating video signals correspondingto imagery in the scanned areas of the photographs; error sensing means,including differential delay means correlating means responsive to saidgenerated signals to determine the magnitudes of any relative timedisplacements therebetween, for developing height error signalscorresponding to said magnitudes to energize said computer so thatpreviously estimated altitudes of the incremental areas being scannedare updated therein for calculating the positions of homologousincremental areas on the photographs be synchronously scanned; and meansfor printing out image detail derived from said video, signals andaltitude information derived from the computer pertaining to the scannedphotographs.
 38. In an automatic map compilation system wherein videosignals are produced by electronically scanning incremental areas ofstereoscopic photographs in a predetermined pattern consisting of apredetermined number of scanning lines, the selection of the areas beingcontrolled by a computer as a function of calculations performed thereinbased on prestored photogrammetric data and altitude data previouslyupdated so as to scan substantially homologous terrain areas, said videosignals being correlated to produce height and tilt error signals, usedto vary the computer selected scanned areas so as to synchronously scansubstantially homologous areas, the arrangement comprising:Y-parallaxcompensating means, including correlation circuitry, for producingsignals in response to Y-parallax in a direction perpendicular to a lineat which said photographs were recorded, by correlating the videosignals corresponding to the areas of the photographs scanned inpredetermined scanning patterns, and predetermined relative timedisplacements being introduced in said video signals so as to producesignals indicative of Y-parallax in said photographs, said Y-parallaxcompensating means further including means responsive to saidcorrelation signal to further vary the relative areas synchronouslyscanned so as to compensate for any parallax therein,
 39. The systemdefined by claim 38 wherein said video signals are time displacedrelative to each other by a delay equal to a multiple of the timenecessary to generate a complete scanning line within said patterns soas to produce said signals indicative of Y-parallax in said photographs.40. The system defined by claim 38 wherein the correlated video signalsproduced by scanning incremental areas selected by said computer forsynchronous scanning which have been relatively displaced in a firstdirection with respect to one another in the parallax determiningdirection are compared with correlated video signals produced byscanning said incremental areas which have been displaced relative toone another in said parallax determining direction but opposite inpolarity from said first displacement, the resultant of the comparedsignals producing said signal indicative of parallax in said photographsin the direction of displacement.
 41. An automatic map compilationsystem for recording data, derived by scanning incremental areas of apair of stereoscopic photographs supplied thereto comprising:supportmeans for supporting and moving a pair of stereoscopic photographs,fixedly supported thereon means for synchronously scanning in apredetermined pattern consisting of a series of sequentially generatedscanning lines, incremental areas of the stereoscopic photographs anddeveloping signals in accordance therewith; means responsive to saidsignals and photogrammetric data related to said photographs forcontrolling said scanning means so that incremental areas representingsubstantially homologous imagery on said photographs are synchronouslyscanned; and means for recording altitude data and photographic detailderived from the synchronously scanned homologous incremental areas ofthe stereoscopic photographs.
 42. The system defined by claim 41 furtherincluding, means for controlling said sequentially generated scanninglines scan to substantially contiguous terrain strips within saidincremental areas, the entire areas being scanned in a preselecteddirection.
 43. The system defined by claim 44 further including meansfor controlling said sequentially generate scanning lines of said seriesso that successive lines scan noncontiguous terrain strips withinopposing halves of each of said incremental areas in accordance with apredetermined pattern, the entire areas being scanned by the completeseries of the sequentially generated scanning lines.
 44. A scanningsystem for scanning a surface comprising:means for electronicallyscanning a surface in a predetermined series of sequentially generatedscanning lines each line, scanning an equal strip of said surface fromone end thereof to the other; and means for deflecting successive linesof said series so that predetermined noncontiguous strips of saidsurface are successively scanned with the entire surface being scannedby said predetermined series of sequentially generated scanning lines.45. A system for scanning a surface comprising:means for electronicallyscanning a surface in a predetermined series of sequentially generatedscanning lines each line, scanning an equal strip of said surface fromone end there to the other; and means for deflecting successive lines ofsaid series so that predetermined strips of opposing halves of saidsurface are successively scanned and so that said surface issubstantially scanned by said predetermined series of sequentiallygenerated scanning lines.